Managing the drought
A review of the evidence of the benefits of native
trees species for shelter on the water regime of
pasture and arable crops
Harper Adams University College
Louise Donnison
March 2012
Managing the drought
A review of the evidence of the benefits of native trees
species for shelter on the water regime of pasture and
arable crops
Harper Adams University College
This review was funded by The Woodland Trust and conducted at Harper Adams
University College. Thanks are due to Ivan Grove, James Waterson and Peter
Kettlewell from Harper Adams for their overall guidance and careful reading of this
review and added improvements, and to Mike Townsend from The Woodland Trust
for the overall steering of this review. Originally written October 2011.
Aerial view of arable farmland in Hampshire complete with hedges and trees: © Peter Dean / Agripicture Images
Managing the drought
Contents
Silage machinery in Dorset: © Peter Dean/Agripicture Images
4
Executive summary
6
1 UK agriculture and Climate change
9
1.1 Introduction
9
1.2 Current UK water shortages
9
1.3 Climate change predictions and UK agriculture
10
1.4 Food Security
10
2 Aim of review and methodology
10
3 Crop water use
11
3.1 Introduction
11
3.2 Evapotranspiration
11
3.3 Factors affecting evapotranspiration
11
4Shelterbelts
17
4.1 Introduction
17
4.2 Shelterbelt types
17
4.3 Shelterbelt design
17
4.4 Farm shelter audit
20
5 Microclimate and tree crop interactions
21
5.1 Introduction
21
5.2 Microclimate and soil moisture
21
5.3 Plant structure
22
5.4 Tree crop environment
22
5.5 Computer modelling
24
5.6 Previous review findings
25
6 UK Evidence
26
6.1 Introduction
26
6.2 Lack of UK evidence
26
6.3 Shelterbelt studies
28
6.4 Agroforestry studies
29
6.5 Computer modelling
31
7 European Evidence
33
7.1 Introduction
33
7.2 Shelterbelt studies
33
7.3 Agroforestry studies
35
8 Temperate zone Evidence
37
8.1 Introduction
37
8.2 North America
37
8.3 New Zealand and Australia
39
9Discussion
41
9.1 Benefits of shelter
41
9.2 Shelterbelts in the UK
42
9. 3 Benefits of shelter on the water regime of crops
42
10 References44
11 Appendix50
Managing the drought
1. Climate change scenarios suggest that the UK will experience extremes of weather with drier
hotter summers and wetter winters. Water is already over abstracted in large areas of southern
England. Climate change scenarios predict this will get worse. Water shortages in the UK and
overseas may increase the level and volatility of food prices. To maintain current levels of agricultural
production and ensure food security water needs to be used more efficiently.
2. The review assesses the benefits of native tree species for shelter on the water regime of pasture
and crops. It draws on evidence from the UK, Europe and other temperate zones. Before the
evidence is presented overviews are given of evapotranspiration, shelterbelt design and crop
microclimate.
3. Evapotranspiration is the process by which water is lost from the agricultural environment. It is a
combination of both evaporation from the soil surface and crop transpiration. Crop transpiration
is the process by which water is lost as vapour from plant tissues though small openings on the
plant leaf. The rate of evapotranspiration is controlled by solar radiation, temperature, wind speed
and humidity. The energy to vaporise water from the plant and soil surface is provided by solar
radiation and to a lesser extent temperature. As evapotranspiration occurs humidity levels increase
around the soil or leaf surface. As the air becomes saturated the process slows down unless water
vapour is removed. The replacement rate is dependent on wind speed and turbulence. Faster wind
speeds will transfer larger amounts of dry air over the soil or leaf surface and therefore remove
saturated air more quickly from evaporative zones, thus increasing evapotranspiration rates. Water
for transpiration is supplied from soil water. When levels of available soil water drop below a certain
value the crop is water-stressed and the lack of water results in a reduction in transpiration and
ultimately crop yield.Vegetation and cultivation practices can also impact evapotranspiration rates.
4. Trees planted on farms can be used for fuel, shelter, sport, landscape enhancement and to encourage
biodiversity. Trees can shelter buildings, crops and animals from wind, sun, rain and snow. Shelterbelts
alter the speed and direction of wind. Porosity and height are the main factors that determine the
level of protection given by a shelterbelt. A dense shelterbelt provides a small area of intense shelter.
A shelterbelt of less than 40 per cent porosity will reduce wind speeds by as much as 90 per cent
and protect an area up to 10 times the height of the shelter. Shelterbelt height determines the
extent of cover: the taller the shelter, the larger the area of cover. Tall shelterbelts with an optimum
porosity of between 40–60 per cent protect an area up to 30 times the height of the shelterbelt
creating shelter suitable for crops. The shelterbelt should be as tall as local conditions allow.
Agri-Food Canada recommend shelterbelt trees that at maturity reach 15m in height. A detailed
farm audit can identify the areas in most need of shelter. Shelterbelts composed of native deciduous
trees are a good choice as they are well adapted to local conditions and have a leafless period which
allows pasture to recover in winter from the adverse effects of shading.
5. The effect of shelter on crop production is a result of multiple interacting factors. Shelterbelts
modify the crop microclimate by reducing wind speeds and increasing daytime temperatures.
Lower wind speeds increase the level of humidity around the transpiring plant surface
slowing evapotranspiration. Increased daytime temperature provides more energy to drive
evapotranspiration. Reductions in wind speed can increase vegetative growth. A well watered crop
protected by shelter may use the same amount of water as a non-sheltered crop, but would have
increased photosynthesis rates and therefore increased water use efficiency. Shelter can reduce
mechanical damage of crops. The introduction of trees into the crop environment can modify soil
structure, therefore impacting crop water regimes. However, trees can shade crops and compete for
water and nutrients, reducing crop yields adjacent to shelter. These yield reductions are dependent
on shelterbelt species, crop type and local conditions.Yield reductions typically occur up to a
distance of 1-2H from the shelterbelt, where H represents the height of the shelterbelt.
6
6. A UK study using artificial shelters showed yield increases of wheat and barley in the years when
the weather was hot and dry. Another UK study showed that artificial shelter reduced mechanical
damage of plants, potentially reducing water loss from damaged tissue.Yields of wheat next to
boundary hedges (within 9m of the shelter) can be halved due to tree shading as found by a UK
study. Soil infiltration rates can be increased up to 5m from deciduous shelterbelts as demonstrated
at Pontbren, Wales. Agroforestry experiments conducted in the UK have demonstrated that crops
and native deciduous trees can be grown together while still maintaining good economic returns.
Careful selection of trees, such as choosing deciduous trees over conifers, can reduce the negative
effects of shelter.
7. Farmers in drier parts of Europe are more likely to plant trees for water conservation. A study in
Italy showed that a shelterbelt of 40 per cent porosity could increase rain-fed durum wheat yields
and reduce evapotranspiration rates by 16 per cent at a distance of 4.7H (H represents height of
shelter) from the shelterbelt. Reductions in evapotranspiration rates due to shelter were measured
up to a distance of 12.7H from the shelterbelt.Yield increases were measured up to 18H from the
shelterbelt. Likewise, a study in the Austrian basin showed that evapotranspiration rates of alfalfa
were lower at a distance of 20m from an 8m hedge, compared to 80m from the hedge. A study in
Denmark observed no yield increase with a shelterbelt of a hazel coppice, but the barley crop was
not water stressed.Vegetative growth was enhanced in the study, but this did not transform into
greater grain yields. In Poland large networks of shelterbelts act as water pumps cooling the air of
large areas of the landscape. Trees, due to high rates of evapotranspiration, humidify the air reducing
crop evapotranspiration rates in adjoining fields.
8. Trees are used as shelter in Canada, USA, Australia, New Zealand, China, Argentina and many
developing countries. The Agri-Food Canada website states that shelterbelts can increase wheat
yields by 3.5 per cent and that figure is greater in drier years. Two Canadian studies showed that
shelterbelts increase overall crop yields, despite an area of reduced yield directly next to the shelter.
One study reported that sheltered crop evapotranspiration rates were 75 per cent of non-sheltered
crops at a distance of 1H (H represents height of shelter) from a shelterbelt. Pruning of tree roots
can reduce yield losses next to shelter. Due to inconsistencies in wind direction yield increases were
not observed in an Australian shelterbelt study.
9. Shelter for livestock on Scottish farms is being promoted
by the Forestry Commission and Quality Meat Scotland.
Shelterbelts with a porosity of 40-60 per cent can have
a beneficial effect on crop yields providing an area of
shelter with reduced wind speeds of up to a distance
of 30H (H represents height of shelter) from the
shelterbelt. Crops and trees will compete for water and
light; this effect frequently disappears within a distance
of about 1to 2H from the shelterbelt. The distance of
1.5 to 9H from the shelterbelt is the area which has the
greatest increase in crop yields. These distances can vary
with year, crop, soil type, shelterbelt design, stage of crop
development and geography. The benefits of shelterbelts
become more pronounced when the plants are water
stressed and wind direction is consistent. The evidence
in this review suggests that under the right conditions
native tree shelterbelts could enable UK crops to use
water more efficiently. Shelterbelts can be viewed as an
insurance policy. They may not provide yield increases
every year, but they can buffer crop production when
extreme weather events strike.
Bumblebee on a goat willow catkin: WTPL/John Duncan
Executive summary
Executive summary
7
Managing the drought
UK agriculture and climate change
1 UK agriculture and climate
change
1.1 Introduction
Climate change scenarios suggest that the UK will experience more extremes of weather with drier
hotter summers and wetter winters. Since agricultural land covers 75 per cent of the UK this is an
area of concern, as it faces the agricultural sector with the challenge of operating within an increasingly
unpredictable natural environment (Knox et al., 2010). Trees have the capacity to stabilise and buffer
agricultural production from these climatic extremes (Herzog, 2000). They have been used throughout
history to provide shelter to crops, animals, soils and buildings and to reduce the risk of floods
(Nisbet et al., 2011). These benefits have been recognised by governments since the mid-1400s, when
the Scottish parliament encouraged the planting of wind breaks to protect agricultural land (Brandle
et al., 2004). Currently shelter belts, hedges and small areas of forests are scattered throughout the
landscape, providing shelter to arable and pasture farming. Trees are now needed even more to help UK
agriculture adapt to climate change (FAO, 1989).
This introductory section will examine the current and predicted water shortages facing UK agriculture
and the implications for food security. The UK needs to consider all strategies that can increase
efficiencies in water use including the beneficial role of shelter provided by trees.
1.2 Current UK water shortages
In the last 50 years the amount of water abstracted from UK sources has increased (Charlton et al.,
2010; Weatherhead and Howden, 2009). Large parts of southern England are currently over-abstracted
(Charlton et al., 2010). These areas are permanently under water stress with reduced levels of soil
water available for crop growth. The UK has already experienced severe drought conditions such as
those of 1976 and 2003 brought on by hot, dry summers. The cool, dry winters in combination with
hot, dry summers in the mid 1980s and late 90s resulted in water supplies being put under severe
stress (Charlton et al., 2010).
Arable farmland at Hucking estate in Kent: WTPL/Andrew Butler
8
Currently, agriculture accounts for 1-2 per cent of UK water use, 40 per cent of which is used for crop
irrigation. However, this figure is misleading as there are large regional differences in water use within
the UK. For example, during the summer months, the agricultural sector accounts for 16 per cent of
water use in eastern England (Thompson et al., 2007). The Anglian region is the largest abstractor of
water for crop irrigation. In this area potatoes and vegetables for human consumption are the largest
recipients of irrigation water (DEFRA, 2011). Currently, 55 per cent of their production takes place in
areas considered as over-abstracted (Knox et al., 2010).
Water shortages create challenges for farmers trying to ensure optimum quality and yields and may be
a condition of winning contracts with certain supermarkets (Weatherhead and Howden, 2009;
DEFRA, 2011). Crop quality is dependent on the timing and volume of water applications. In the
drought of 1995 the yield and quality of potatoes, sugar beet and field vegetable crops were reduced
due to irrigation restrictions. Farmers bought extra animal feed to supplement drought-induced
reductions in pasture yield (Charlton et al., 2010).
9
Managing the drought
1.3 Climate change predictions and UK agriculture
Farming will face a challenging future. Water shortages are predicted to get more acute, exacerbated
by legislative restrictions (e.g. the 2003 Water Act in the UK) (Morrison et al., 2008). UKCP09 climate
change scenarios predict that temperatures will increase in the UK, particularly in summer. In summer
southern England is predicted to have an average increase in temperature of 4ºC, whilst the Scottish
islands will warm by an average of 2.5ºC. The far south of England is expected to experience summer
droughts, as there will be a 40 per cent reduction in rainfall and a decrease in humidity of about
9 per cent. In winter the western side of the UK is predicted to get 33 per cent more rain
(Murphy et al., 2009). No projections were made for snow fall. There is projected to be very little
change in wind speed (DEFRA, 2010a).
The effects of climate change could be more marked than the averages suggest. Different land use,
topography and soils will generate variability in water quality and quantity (Weatherhead and Howden,
2009). Irrigation is likely to become more important, with existing irrigated crops requiring more water.
Rain-fed crops such as wheat are likely to be under increasing levels of water stress with less available
soil water. To maintain current livestock stocking levels, grass may have to be irrigated. Crop production
could move north, where water will be more available. The price of water for agricultural production
is predicted to increase under climate change predictions. Farmers will be under increasing pressure to
maintain current production levels (Charlton et al., 2010; Knox et al., 2010).
1.4 Food Security
There will be less available water under climate change scenarios. This has the potential to increase the
level and volatility of food prices. This is significant, as the UK is an importer of ‘virtual water’, relying
heavily on imports of agricultural products. Extreme weather events may create disruptions in food
production and distribution. Agricultural production needs to be maintained at current levels to ensure
UK food security. To ensure this water needs to be used more efficiently by crops (Thompson et al.,
2007; DEFRA, 2010b). Irrigation may not be a viable answer to climate change if constraints are placed
on UK agricultural water usage (Weatherhead and Howden, 2009).
2 Aim of review and
methodology
UK agriculture derives many benefits from native trees. They may help UK agriculture adapt to current
and future water shortages. This review will present evidence to suggest that shelterbelts composed of
native trees species can have a beneficial effect on the water regime of pasture and arable crops.
10
WTPL/Allan Baxter
The literature search for the review was conducted by predefining keywords and using them to search
the online database Google Scholar. Other online databases were searched using a subset of keywords.
Keywords and all search sources for the literature review can be found in Appendix 1. Peer reviewed,
grey literature and government reports were all considered relevant.
Dairy cows at pasture near Llangathen; WTPL/Colin Baglow
The report is divided into two main sections. The first section is theoretical. It desccribes evapotranspiration, the process by which water is lost from the crop environment, then gives a
description of shelterbelt design and finishes, and then describes the mechanism by which shelterbelts
modify the crop microclimate to increase crop water use efficiency. The second section introduces
evidence to suggest that native trees may help UK agriculture adapt to current and future water
shortages. It draws on evidence from agricultural systems of the UK, Europe and other temperate
zones. Despite the lack of UK research in this field, evidence from comparable countries can provide
valuable insight into the potential of trees to combat the adverse effects of climate change.
11
Managing the drought
Crop water use
3 Crop water use
3.1 Introduction
UK agriculture is facing water shortages. By targeting the mechanism by which water is lost from
crops, water savings can be made. This section of the report covers the fundamentals of crop water
loss through the process of evapotranspiration. The rate of evapotranspiration is driven by climatic
conditions, vegetation and soil type. By favourably modifying these climatic conditions, crop water
savings can be made
3.2 Evapotranspiration
Evapotranspiration is the process by which water is lost from the agricultural environment. It is a
combination of both evaporation from the soil surface (evaporation) and crop transpiration.
Crop transpiration is the process by which water is lost as vapour from plant tissues through small
openings on the plant leaf called stomata. Stomata control the rate of evapotranspiration by modifying
their aperture size (Allen et al., 1998). Water for transpiration is supplied from soil water. Water is taken
up through the roots, transported through the plant and lost at the leaf surface by transpiration. These
losses are considerable, with over 90 per cent of water taken up by plants lost through transpiration
(Morrrison et al., 2008). The two processes involved in evapotranspiration occur at the same time.
Before a crop creates a full canopy much of the water loss comes from evaporation from the soil.
As the plant canopy increases then the loss from transpiration becomes dominant (Allen et al., 1998).
3.3 Factors affecting evapotranspiration
The weather parameters that affect the rate of evapotranspiration are solar radiation, temperature,
wind speed and humidity. Soil type, organic matter content, soil water content and soil structure plus
cultivation type and depth all contribute to the amount of water available for plant root uptake and
thus affect the rate of evapotranspiration. Evapotranspiration rates also vary with vegetation type,
density, developmental stage, leaf surface roughness and reflective capacity, rooting depth, density and
root architecture.
3.3.1 Climatic conditions
The climatic factors involved with evapotranspiration are those which provide energy for the
vaporisation of water and those that remove water vapour.
Vapour generation (Solar radiation and air temperature)
Potato irrigation: © Peter Dean/Agripicture Images
12
The energy to vaporise the water from the plant and soil surface is provided by solar radiation.
The amount of energy available in radiation depends on latitude, season and weather. Solar radiation
levels at the plant and soil surface will be lower on cloudy days. As the crop grows it shades the soil
resulting in less radiation reaching the ground. Therefore, as the plant grows soil evaporation decreases
and crop transpiration increases. The air temperature surrounding the crop drives evapotranspiration
to a lesser extent. As weather becomes warmer and less overcast evapotranspiration and crop water
requirements increase (Allen et al., 1998).
13
Managing the drought
Vapour removal (Vapour pressure humidity and wind speed)
As evapotranspiration proceeds humidity levels (vapour pressure) increase around the soil or leaf
surface. As the air becomes saturated the process slows down unless the water vapour is removed.
The replacement rate is dependent on wind speed and turbulence. Faster wind speeds will transfer
larger amounts of dry air over the soil or leaf surface and therefore increase evapotranspiration rates.
In hot dry conditions variations in wind speed can result in large variations in evapotranspiration rate,
as compared to cooler weather conditions (Allen et al., 1998). Any mechanism that reduces wind speed
will also reduce evapotranspiration rates. Shelterbelts of trees can reduce wind speeds and offer the
potential to reduce the evapotranspiration rates.
for the heavier canopies of oak or beech.Trees growing on chalk based soils may be able to maintain water
uptake during periods of drought, by capillary rise of otherwise unavailable water (Nisbet, 2005).
Evapotranspiration rates of arable crops are higher than those of most trees, however yearly average
rates are lower due to the short cycle of crops (Nisbet, 2005). The annual evaporation losses for
arable crops are 370–430mm, as compared to 400–640 mm a year for deciduous trees. These figures
are based on the land surface receiving 1000 mm annual rainfall (Nisbet, 2005). Crop type, variety and
development stage area all factors which will affect evapotranspiration rates. Differences are a result of
variations in crop height, root depth and leaf anatomy.Variations in rates of evapotranspiration of a crop
at the same developmental stage within a field are low (Allen et al., 1998; Blyth et al., 2006).
3.3.2 Soil water content
3.3.4 Measuring evapotranspiration
Rainfall, irrigation and capillary rise of groundwater all add water to the soil root zone. Soil evaporation,
crop transpiration, run-off and drainage remove water from the root zone.
Evapotranspiration can be measured in the field with lysimeters, or calculated using the PenmanMonteith equation using weather data. A lysimeter is an isolated tank filled with moist soil in which
a crop is grown. The rate of evapotranspiration is measured by the change in mass of the lysimeter.
The vegetation inside and outside the lysimeter should be identical (height and leaf area). The PenmanMonteith equation is based on a wide range of climatic and location data and can then be modified with
crop-specific details. Daily mean temperature, wind speed, relative humidity and solar radiation are all
parameters used in the calculation. Therefore, if wind speed alone is reduced the Penman-Monteith will
calculate a lower rate of evapotranspiration (Allen et al., 1998; Cleugh, 1998). However, the reduction of
wind speed may also affect the temperature and relative humidity around the crop canopy and thus the
effect of wind speed cannot be considered in isolation.
How water is added to the soil
As water is applied to soil in the form of snow, rain or irrigation, it is absorbed by the ground if the
soil water is lower than field capacity. This process is called infiltration. The rate of infiltration depends
on soil texture; it is faster for sandier, coarse-textured soils and slower for fine-textured clay soils.
Water infiltrates faster when the soil is dry and not compact. When water is first applied to dry soils
it infiltrates easily, but as the soil becomes wet, the infiltration rate decreases as the soil pores fill with
water. Once the soil pores are filled with water and there is no air left in the soil it becomes saturated.
Some of this water moves downwards under gravity and as these pores become saturated the water
moves beyond the root zone to deeper soil layers. These layers may be permanently saturated and
form the water table. When all drainage ceases the soil is said to be at field capacity, an optimum state
for plant growth. As the soil begins to dry the ground water can move upwards through small pores
(capillars) this process is called capillary rise. This rise can be extensive, up to 2m in clay, but is relatively
small, 50cm, in sands. For crop production, capillary rise is not considered as a useful water source
except for particular situations. Infiltration and soil water holding capacity can be increased by cultural
practices that favourably modify the soil structure or increase the organic matter content of the soil.
Trees through root exploration modify soil structure and therefore offer the potential to increase soil
infiltration rates (Allen et al., 1998; Brouwer et al., 1985)
How water is removed from soil
Water stored in the soil is used by plant roots. Near to the surface it is also evaporated from the
topsoil. Too much soil water can result in waterlogging which damages roots and inhibits plant
respiration. As soil dries water becomes more strongly bound to the soil matrix and becomes less
easily available to the crop. Evaporation rates then decline as there is not enough water to supply
the evaporative process. When contents of available water fall below a crop-specific critical value
the crop becomes water-stressed and the lack of water results in a reduction in transpiration and
thus productivity. The total water content in the soil is the difference between permanent wilting
point (point where the plant wilting is irrecoverable) and the ‘field capacity’ of that soil. The term ‘soil
moisture deficit’, SMD, is used to quantify the amount of water needed to raise the soil water content
back to field capacity. Therefore the soil moisture deficit can exert a controlling effect on the rate of
evapotranspiration (Allen et al., 1998).
3.3.3 Vegetation type
During rainfall water is lost through evaporation from the branches and leaves of trees due to their height
and rough aerodynamic profile, a process called interception. Interception rates vary amongst deciduous
trees. Ash and birch lose about 11 per cent of water through interception as compared to about 15 per cent
14
Crop water use
3.4 Mechanisms to reduce crop water loss
As discussed in the introductory section, large areas of southern England are expected to become
hotter and drier. Knox et al. (2010) used UK climate change data generated for 2050 (UK Climate
Impacts Programme) to model rainfall, evapotranspiration and soil moisture deficit under climate
change scenarios. Knox et al. (2010) calculated that by 2050 there would be a 20-30 per cent increase
in aridity in summer (April to September) assuming current summer evapotranspiration rates of
3mm/day. This has serious implications for UK agriculture as it will need to improve its crop water
use efficiency to maintain current production levels. Improvements in crop drought resistance and
agricultural practice will increase crop water use efficiencies.
3.4.1 The crop
Conventional breeding programmes can be used to select water-efficient crops that are drought resistant.
Cultivars that give higher yields under conditions of water stress should be selected. Antitranspirants sprayed
onto wheat leaves to cover stomata at the drought sensitive stage may produce water savings (Kettlewell,
2010). Trials could be established in southern England to investigate the viability of growing Mediterranean
crops more suited to hotter climates, such as olives and apricots (Thompson et al., 2007; Charlton et al., 2010).
3.4.2 Agricultural management
Improvements in irrigation use and scheduling will produce water savings. Measures to reduce soil
compaction can also significantly enhance soil water content (Thompson et al., 2007). Mediterranean
studies suggest that by combining both crop breeding programmes with improvements in agricultural
husbandry water savings can be made (Morrison et al., 2008).
15
Shelterbelts
The benefits of native trees species for shelter on the water
regime of pasture and arable crops
4 Shelterbelts
4.1 Introduction
Traditionally trees have been used to shelter buildings, crops and animals from wind, sun, rain and snow.
The UK is a windy country, so by reducing wind speeds shelterbelts offer the potential to improve
agricultural production (Gardiner, 2004). Under climate change predictions evapotranspiration will
increase, thus the shelter offered by trees may be beneficial for reduced water use.
This section introduces the different shelter formations trees can provide, how shelterbelts work and
factors such as height and porosity that determine their usage. The benefits derived from shelterbelts
can be maximised if a detailed farm audit is done to find areas of most need on a farm.
4.2 Shelterbelt types
Trees are used for a variety of purposes on a farm. The combination of agriculture and forestry is
termed agroforestry. By combining the two systems ecological and economic benefits can be gained
(Sinclair, 1999).
Trees can be planted in different configurations to provide shelter.Vigiak et al. (2003) describe a system
devised by Drachenfels that divided shelter into the following six categories of hedgerow, hedge, line
of coniferous trees, single line of deciduous trees, multiple lines of deciduous or mixed trees and small
wood. This review will use the term shelterbelt to encompass all forms. However, Gardiner et al. (2006)
preferred the term shelter wood suggesting that trees are not always in a linear formation that a
shelterbelt may imply.
4.3 Shelterbelt design
Shelterbelt protecting young crops: © UKagriculture
16
Shelterbelts alter the speed and direction of wind (Brandle et al., 2004). A shelterbelt obstructs the flow
of air upwind (windward) and downwind (leeward). As wind approaches the shelterbelt it can flow in
three directions, either through the shelterbelt, around the sides or upwards (Cleugh, 1998).
Above the shelterbelt there is increased turbulence and wind speed - this is called the displacement
zone. The shelterbelt creates an area of high pressure and turbulence on the windward side which
is one of the factors forcing the air upwards. On the lee side there is an area of low pressure. If the
shelterbelt is dense enough on the lee side, the air stagnates resulting in increased turbulence.
The area either side of the shelterbelt containing turbulent air is termed the cavity zone. Beyond the
cavity zone on the leeward side the faster moving air from the displacement zone combines with the
slower moving air flowing through the shelterbelt to create the wake zone. Eventually downwind of the
barrier the effects of the wind break are lost and the wake zone ends (Brandle et al., 2004; Gardiner,
2004; Gardiner et al., 2006). Cleugh and Hughes (2002) substantiated the existence of these zones by
wind tunnel experiments. Figure 1 shows the different shelterbelt zones and effects on wind speed
and turbulence.
17
Managing the drought
Shelterbelts
4.3.1 Shelterbelt performance
4.3.3 Height
The effects of shelterbelts are measured in H, where H is the height of the shelterbelt. This review will
use the H notation to describe the results of shelterbelt experiments. The upward movement of air
can create a region of reduced wind speed on the wind ward side from 2 to 5H from the shelterbelt.
A larger area of reduced wind speed is created on the leeward size which can range from 10 to 30H.
These distances are modified by the porosity and height of the shelterbelt. Other factors such as length,
width, orientation and surrounding landscape will all effect the size of the area protected (Brandle et al.,
2004; Gardiner, 2004; Gardiner et al., 2006).
Shelterbelt height determines the extent of cover; the taller the shelter the larger the area of cover.
It is normally measured from the centre of the shelterbelt (Gardiner, 2004; Gardiner et al., 2006).
The shelterbelt should be as tall as local conditions allow, Agri-Food Canada recommend shelterbelt
trees that at maturity reach 15m in height (AAFC, not datedb).
4.3.4 Width
Width is important because it affects porosity. Porosity can be decreased by adding another row of
trees to the shelterbelt. In windy locations the addition of another row of trees to a shelterbelt can
ensure that the shelterbelt has the desired height as the most windward row of trees is often stunted.
However, wider groups of trees tend not to provide large areas of shelter and take up more space.
Clear objectives need to be set for the shelterbelt , which will then drive shelterbelt design, for example
whether the wood will be used for timber or fuel (Gardiner, 2004; Gardiner et al., 2006; QMS, 2011a).
4.3.5 Length
To reduce the effects of wind flow around the ends of the shelterbelt, it is recommended that the
length of the shelterbelt should be at least ten times the height. This results in as area of cover the
shape of a triangle with the edges of the covered area returning to the local wind speed quicker that
the centre (Figure 2) (Gardiner, 2004; Gardiner et al., 2006).
Figure 1
Wind speed and turbulence in shelterbelt zones. Source: after Gardiner et al. 2006.
An opening in the shelterbelt can result in increased wind speeds which has the same effect as the
edge of a shelterbelt. If an opening is needed for access then it should be angled into the shelterbelt
(Gardiner, 2004; Gardiner et al., 2006).
4.3.2 Porosity
Porosity is how easily wind can flow through the shelterbelt. It influences the level of protection
provided to the leeward side of the shelterbelt. In general, the less porous the shelterbelt the more
intense but shorter the protection. Porosity is affected by tree density, species and the time of year.
If the shelter is too dense it can increase turbulence in the cavity zone, resulting in increased wind
speed (Gardiner, 2004; Gardiner et al., 2006).
Optical porosity has traditionally been used to give an estimate of the porosity of shelterbelts
(Cleugh, 1998). More recently aerodynamic porosity has been used to measure porosity as the
shelterbelt can be considered as a three-dimensional structure. Nelmes et al. (2001) measured wind
speed upwind and downwind of different types of shelter. They produced a calculation that could assess
the effectiveness of different shelters giving an indication of aerodynamic porosity. Brandle et al. (2004)
concluded that although optical porosity can overestimate aerodynamic porosity, optical porosity was
still a valid indicator of shelter performance. Porosity is a key measurement for modelling programmes,
but can be particularly difficult to assess for wide rows of deciduous trees (Vigiak et al., 2003).
Deciduous trees will keep 60 per cent of their sheltering effect in winter which is enough to provide
adequate shelter (Palmer et al., 1997).
18
Field
Hedge
Wind
direction
Area sheltered
30H
Figure 2
Area of shelter behind a shelterbelt. Source: after Gardiner et al. 2006.
4.3.6 Orientation
Orientation of the shelterbelt is important as it affects the area covered. The greatest cover is when the
wind hits the shelterbelt at right angles, but is reduced when this angle is decreased. If the wind does
not hit at right angles then the porosity of the shelterbelt decreases and the area of cover is decreased.
This effect is more pronounced with wider shelter belts (Gardiner, 2004; Gardiner et al., 2006).
19
Managing the drought
4.3.7 Air turbulence and surrounding area
Weather, topography and the surrounding vegetation all affect air turbulence. To manage gusts around
hills, shelterbelts may be needed on more than one side. Wind tunnels can be formed between lines of
trees or valleys. Thus, the local area should be considered when designing shelterbelts (Nelmes et al., 2001;
RHS, not dated).
Shelterbelt networks impact air turbulence. Shelterbelt systems can slow down the average wind speed of
a whole region. Increased roughness of vegetation can create turbulence, for example a sugar beet crop is
not as rough as grass and thus creates less air turbulence (Vigiak et al., 2003; Tuzet and Wilson, 2007).
Microclimate and tree crop interactions
5 Microclimate and tree
crop interactions
5.1 Introduction
4.4 Farm shelter audit
As discussed in Section 3, solar radiation, temperature, humidity and wind speed all affect the rate
of evapotranspiration. The previous section described how shelterbelts can reduce wind speed
(Gardiner, 2004). This reduction in wind speed can improve the crop microclimate and make the crop
more efficient in its water use. This section will examine the effect a shelterbelt has on crop water use
including both beneficial and negative effects.
4.4.1 Match porosity to use
5.2 Microclimate and soil moisture
It is important to be clear on the objectives of a shelterbelt. A dense, low porousity shelterbelt
provides a small area of intense shelter. A shelterbelt of less than 40 per cent porosity will reduce wind
speeds by as much as 90 per cent and protect an area up to ten times the height of the shelter.
They are suitable for protecting buildings and young livestock. Tall shelterbelts of 40–60 per cent
porosity protect an area up to 30 times the height of the shelterbelt creating suitable shelter for crops.
Hybrid shelterbelts which are dense at the base and more porous at the top can perform two
functions. They provide a wide area of shelter, but at the base a very sheltered area.Very porous
shelterbelts of more than 60 per cent porosity can increase wind speeds. Porosity can be increased by
thinning and pruning the trees and reducing the width of the shelterbelt. Porosity can be reduced by
under-planting, fencing and straw bales (Palmer et al.,1997; Gardiner, 2004; Gardiner et al., 2006).
4.4.2 Farm shelter audits
A farm shelter audit was created by the Forestry Commission and SAC (Scottish Agricultural College).
It provides a framework with which to advise farmers on the creation of shelter around a farm. Trees can
be of benefit to farmers, but only in the right location and layout. The farm shelter audit consists of three
steps. The first step involves gathering information to establish the potential benefits and assess existing
shelter effectiveness. Information is gathered on farm size, existing boundaries, activities taking place in
each field and direction of problem winds. Local weather conditions are considered such as location of
frost pockets, frequency of droughts and soil erosion problems etc. Shelterbelts should face the prevailing
wind, often from the south-west in the UK, but colder winds can come from the north east (RHS, not
dated). Further information on wind speed directions can be gained from the governmental wind speed
database (DECC, not dated). Once all this information is gathered recommendations can be given as to
whether shelter would be beneficial and if so what type (Hislop et al., 1999; QMS, 2010).
If shelterbelts are to be planted the amount of land available and the ultimate use of the trees needs to
be considered. Hedges do not take much space and can be thinned to desired porosity (QMS, 2010).
Altitude, hydrology, soil type and pH are all important factors to consider in the species selection of
the shelterbelt. Native trees already growing in the area are often a good choice (QMS, 2011a; QMS,
2011b). The Woodland Trust can provide guidance on tree and site selection (The Woodland Trust, not
dated; Forestry Commission, not dated).
5.2.1 Wind speed and humidity
In the wake zone of a shelterbelt the movement of air is slowed above the crop canopy. This reduces
the rate of water vapour removal from the crop surface, resulting in the build-up of a moisture layer
around the crop. The air becomes more humid and the rate of evapotranspiration declines. Shelterbelts
also slow the transfer of heat (heat advection) to the crop. Shelterbelts reduce the amount of hot air
moving over the transpiring crop surface and thus the rate of evapotranspiration. Reductions in wind
speed can occur up to 30H from the shelterbelt (Kort, 1988, Gardiner, 2004.)
5.2.2 Temperature
In the wake zone of the shelterbelt daily air temperatures can increase by as much as 4°C due to
reductions in wind speed, which results in higher leaf surface temperatures. This increases transpiration,
photosynthesis and respiration. At night, temperatures can be reduced by up to 1°C due to air not
mixing resulting in reduced respiration and increasing growth overall (Kort, 1988; Brandle et al., 2004).
Using wind tunnels, Cleugh and Hughs (2002) confirmed that wind speed reductions occurred up to 30H
downwind of a shelterbelt, whereas the temperature effect was up to 10H.They found that at 20H wind speed
could be 80 per cent of the upwind value. Figure 3 summarises the main climatic effects on each shelter zone.
5.2.3 Soil water and irrigation
Reductions in wind speed reduce
soil moisture evaporation rates
and can improve the distribution
of irrigation water. However,
increased soil moisture can drive
evapotranspiration, as there is
more water available to the plant
to transpire (Davis and Norman
1988; Dicky, 1988).
Figure 3
Shelterbelt zones and microclimate: source after Gardiner et al. (2006).
20
21
Managing the drought
5.2.4 Water use efficiency
5.4.1 Tree crop interactions
Shelterbelts can reduce evapotranspiration rates by increasing humidity levels and decreasing heat
advection. Evapotranspiration rates in the lee of a shelterbelt can increase as daytime air temperatures
are higher. The overall net effect on evapotranspiration is determined by such factors as light,
temperature and moisture stress acting on stomatal aperture size. Stomata in sheltered plants may
stay more open, which allows more carbon dioxide exchange, resulting in increased photosynthesis
and growth rates. Thus, when comparing evapotranspiration rates net gains in biomass should be
considered. The comparison of the ratios can give an indication of crop water use efficiency
(Nuberg, 1998; Davis and Norman, 1988).
Species compete if they are in the same environment and have the same requirements for resources.
Trees modify the amount of light, water and nutrients available for crop growth. This zone of interaction
can extend up to a distance of about 1H to 2H from the shelterbelt (Nuberg, 1998). Plants can coexist
if the biomass gains are greater than if they are grown separately (Jose et al., 2004; Kort, 1988).
5.3 Plant structure
5.3.1 Plant growth
Increases in photosynthetic rate will occur up to about 30°C. This results in longer growing season and
a greater growth rate. Plants growing within shelter are often more luxuriant and can have a larger leaf
area index than those in exposed areas. However, this greater leaf area index can increase transpiration
losses. Furthermore, increased temperatures in sheltered areas can reduce the length of developmental
stages, thus reducing time for photosynthesis. Increased growth may not equate to larger grain yields.
Crops grown for biomass may yield greater returns than those harvested for grain.
(Allen et al., 1988; Kort, 1988; Cleugh, 1998). Cleugh (1998) concluded that a well-watered crop
protected by shelter would use the same amount of water as a non-sheltered crop, but would have
increased photosynthesis rates and increased water use efficiency.
5.3.2 Mechanical
Plants growing in sheltered areas experience less wind damage. If plants have wind damage they can
lose water from the damaged tissues (Grace, 1988). Cleugh and Hughes (2002) found using wind
tunnels that the benefits of shelter extended 30H from the shelterbelt, whereas those for temperature
extended to about 10H. From this they concluded that the benefits of reduced mechanical damage
would extend longer than those of improved microclimate. Those benefits can start from the initial
sowing of the crop. Seeds that are planted at shallow depth are less likely to be blown out of the soil.
A shelterbelt can protect seedlings from sand blasting by soil particles as the crop grows plants will be
less likely to be knocked against each. Wind can also increase lodging and thus crop damage.
Horticultural products respond particularly well to shelter as fruits will be produced earlier and are
less prone to damage (Nuberg, 1998; Cleugh, 1998; Baldwin, 1988).
5.4 Tree crop environment
Living shelters also have an impact on the crop environment. Tree root growth can improve soil
structure aiding water infiltration and reducing water run-off. Tree transpiration can create a moisture
blanket over crops reducing crop transpiration. However, trees can compete with crops for water and
reduce photosynthetic rates by shading crops.
22
Microclimate and tree crop interactions
Competition for water
The highest density of tree and crop roots occurs in the top 30 cm of soil (Jose et al.,2004). Lack of
deep ploughing before tree planting, compact soils and low soil water moisture will all encourage
competition between crops and tree roots. Competition for water and nutrients will be reduced if
there is spatial separation of tree and crop roots (Jose et al., 2004).
Shading effect of trees
Shading reduces the amount of solar radiation the understorey crop receives, which can slow down
the rate of photosynthesis and transpiration (Benavides, 2009; Jose et al., 2004). The orientation and
crown density of the shelterbelt will influence the degree of shading. Shelters running north-south minimise the effect of shading (Natural England, 2008).Young ash and sycamore do not heavily shade
crops, as they grow initially upwards and only develop their branches horizontally as they mature
(Hein and Spiecker, 2008). Deciduous trees do not have a detrimental effect on pasture production
until there is 85 per cent canopy cover as compared to 67 per cent for conifers. The leafless period
of deciduous trees enables pastures to recover from the adverse effects of shade and can encourage
pasture growth in spring. Pruning of lower tree branches reduces the effect of shading and results in
knot-free timber (Sharrow,1999; Benavides et al., 2009).
5.4.2 Water infiltration
Tree root exploration and increased litter inputs can improve soil organic matter and thus infiltration
rates and water holding capacity. Animals congregating by a shelterbelt can increase soil compaction
and reduce soil infiltration rates. Trees can prevent water reaching the crop through canopy
interception; the amount of rain intercepted is dependent on tree density and crown size. Under heavy
rainfall conditions this can prevent the impact of raindrops on the ground damaging soil structure.
Shelterbelts can create rainshadows which can extend 1H on the lee side of the shelterbelt (Bird, 1998;
Riha and McIntyre, 1999; Jose et al., 2004; Chandler and Chappell, 2008).
5.4.3 Hydraulic lift
Due to their deep roots, trees can intercept water unavailable to shallower rooting crops through
hydraulic lift. This is the movement of water from more moist soil layers to drier soil layers via plant
roots. Water lifted by plant roots is released at night when transpiration ceases, this water becomes
available during the day for transpiration. An experiment over five days showed that a mature 20m
tall maple tree can lift about 102l of water a night (the review did not state in which season the
experiment was conducted). The water added to the upper soil layers accounted for 25 per cent of the
transpiration losses (Caldwell et al., 1998). If tree roots access water below the crop rooting zone, then
tree crop water competition will be reduced (List and White, 2008). Whether the water lifted by trees
becomes available to the crop is not known (Jose et al., 2004).
23
Managing the drought
Microclimate and tree crop interactions
5.4.4 Tree transpiration
As the wind passes through shelterbelts, the water vapour generated by tree transpiration humidifies the
air. Moisture blankets can develop over the crop canopy, resulting in reduced rates of crop transpiration.
5.4.5 Allelopathy, nutrient cycling, insects and pathogens
Trees and crops will compete for nutrients. Due to their deeper roots, trees can access nutrients at
greater depths and may reduce nutrient leaching. Tree litter on the ground can buffer against soil water
losses. Tree roots can exude toxins that reduce plant growth; this process is termed allelopathy. Insects,
pollen and pathogens use wind as transport, thus shelter will modify these pathways. In orchards there
are often increased numbers of insects which aids pollination. However, the reduction in wind speed
can result in pests and pathogens being deposited on the lee side of the shelterbelt. Modification of
humidity levels can also impact fungal diseases (Bird, 1998; Eichhorn et al., 2006; Cleugh, 1998; Jose et al.,
2004; Kort, 1988).
5.5 Computer modelling
Advantages
Increased air temperature resulting in improved crop
germination and growth
Extension of growing season due to increased soil and
air temperatures
Increase in lodging in the cavity zone
Control of snow drifting, spray drift and irrigation water
Reduction in pollination for some
crops
Reduced mechanical damage leading to less water loss
from plant tissues
Reduction of crop lodging
Reduction of soil erosion
Increased soil organic matter and nutrient recycling
from leaf litter
Mineralization of soil nitrogen encouraged
Reduction of soil acidification for certain soils
Increased pollination for some crops
Table 1
Advantages and disadvantages of shelter: source Gardiner, 2004.
24
Disadvantages
Trees compete with crops for
light, water, and nutrients resulting
in reduced crop yields close to
shelterbelts
Reduction in soil evaporation and plant transpiration
Farmland at Hucking Estate: WTPL/Nicholas Spurling
Table 1 summarises some of the advantages and disadvantages of shelterbelts. Computer models can
help unravel the complexities of crop shelterbelt interactions. By modifying climatic parameters such as
rainfall and temperature the potential benefits of shelterbelts could be predicted under climate change
scenarios. However, computer models are prone to being over simplified (Mize et al., 2008b; Palma et al.,
2007; Jose et al., 2004).
Land taken out of production and
added cost of establishment and
maintenance.
Potential of water logging of soil
close to low porous shelter belts
5.6 Previous review findings
The most tangible benefit that shelterbelts bring is increased yield. As this section has demonstrated
the exact benefit a shelterbelt provides may be harder to determine. There are a number of reviews
that cover this topic in more detail. Kort (1988) and Nuberg (1998) looked at the effect of shelter
on arable crop yields, whilst Bird (1998) reviewed the literature for the effect of shelter on pasture.
These reviews concluded that a large number of studies have shown improved growth and yield over
a range of climates and soils, although the results were variable depending on crop, weather, year
shelterbelt type and soil.Yield increases are more consistent for vegetables as detailed in the review
of Baldwin (1988). Kort (1988) noted that quantitative comparisons between studies were difficult as
many parameters were not recorded. Often it was not clear if the land taken out of production for the
shelterbelt was deducted from the yield calculations (Cleugh, 1998). Shelterbelt studies should have
detailed microclimate measurements at varying distance from the shelterbelt (Grace, 1988; Nuberg,
1998). The next three sections will review the evidence for the benefits of native trees species for
shelter on the water regime of pasture and arable crops.
25
Managing the drought
Forest cover in the UK
6 UK evidence
6.1 Introduction
Trees planted on farms can be used for fuel, shelter, sport, landscape enhancement and to encourage
biodiversity. Trees were planted in large parks to enhance the aesthetic appeal of the landscape (Sibbald,
2001). Woodland edges can provide habitats for game hunting. Farmers can diversify their income by
producing timber which can be sold or used for fencing and fuel (The Woodland Trust, not datedb).
Until 1989, Bryant and May intercropped poplar with crops for match stick production; when the trees
became larger the understorey was replaced with pasture (Lawson, et al, 2004; Sibbald et al., 2001).
Shelter can have a number of purposes. Trees such as poplars or alders have been planted to provide
shelter to orchards in Kent (Natural England, not dated). Breadalbane Farm Forestry and Glensaugh
run silvopasture demonstration sites (Breadalbane, not dated; Glensaugh, not dated).The Forestry
Commission and Quality Meat Scotland have a current initiative promoting farm woodlands to provide
shelter to livestock (QMS, 2010; QMS, 2011a; QMS 2011b).
Hedges to define boundaries of fields and to prevent movement of livestock have a long tradition in the
English landscape. Many hedges are very old. In Devon, it is believed that over a quarter of the hedges
are more than 800 years old (Hedgelink, 2009). Hedges were planted to shelter horticultural crops in
coastal areas such as Cornwall, Isle of Scilly, or in exposed regions such as the Wirral, South Hampshire
and West Sussex (Hooper and Holdgate, 1968). Oak and ash are the most common hedgerow trees,
but beech can also be found. For example, in the 19th century beech hedgerows were planted at
Exmoor. In 1998, a survey showed that less than 1 per cent of hedgerows were younger than four years
old, highlighting the need to encourage new plantings (Natural England, 2008).
6.2 Lack of UK evidence
There has been very little UK research on the benefits of shelter to crops and pasture. Bird (1998)
commented that there was a lack of research globally on the benefits of shelter on pasture growth. Bird
(1998) cited only one UK study, that of Russell and Grace (1979). Likewise, Nuberg (1998) in a review
of the literature on the benefits of shelter to arable crops mentions only one UK study, that of Hough
and Cooper (1988). Kort (1988) reviewed 50 years of shelter literature and made no mention of any
UK research.
Two UK studies have examined the effect of artificial shelter on pasture and crop growth. UK
agroforestry initiatives have demonstrated that trees intercropped with pasture or arable crops can
yield good economic returns. The following section will present the findings of these studies.
26
Hedgerow shelterbelt at Pontbren: WTPL/Rory Francis
Caborn (1955) pioneered UK shelter research, studying the microclimate of Scottish shelterbelts.
These shelterbelts planted in the 19th Century have become neglected (Hislop et al., 1997). In the
1970s there was a series of shelter symposiums organised by MAFF (Gloyne, 1976). Gloyne (1976)
reports on the work of Alcock who found that shelter provided early grazing of pasture in Welsh hills.
He cites the work of Drew who showed that shelter aided carrot germination as sand blasting was
reduced. Horticultural produce in the UK also prospers under shelter. For example, Gloyne (1976)
reported that strawberries were grown successfully in Scotland when sheltered.
27
Managing the drought
6.3 Shelterbelt studies
6.3.1 Arable
Hough and Cooper (1988) conducted experiments in north-east England examining the effect of artificial
shelter on yields of spring barley, winter wheat and winter barley. The shelter had a porosity of 50 per
cent, and was 0.9m tall and 80m long. Measurements were made behind the shelter at various distances in
plots up to 20m or 30m from the shelter. Sheltered crops were taller and yielded more grains than nonsheltered crops for the first two years. In the third year results were inconclusive. The authors attributed
these results to climatic conditions. In the first two years there were periods of dry windy weather for
several weeks before anthesis, whereas in the third season it was wet with mild winds until anthesis.
Assuming the benefits of shelterbelts extend up to 30H into the field, then it could be hypothesised that a
5m hedge of 50 per cent porosity angled against the predominant winds could afford protection of up to
150m into a field. Likewise a 10m high shelterbelt could protect an area up to 300m long. These values are
absolute maximums and would need to substantiated by further field experiments.
The authors suggested that the beneficial effect of shelter was to reduce evapotranspiration. Using
the Penman-Monteith equation they calculated that shelter would decrease transpiration by 10 per
cent. They suggested that during drier periods the soil moisture levels may be higher under sheltered
treatments. They supported this by field observations, noting that after light rain fall the sheltered top
soil was not as dry as the unsheltered top soil.
Sparkes et al. (1998) investigated the effect of field margins on crop yield. Over five the years, field margins
of commercial farms growing sugar beet, wheat and barley were studied. They found that there was a
reduction in yield at the edge of the field and this was most pronounced for sugar beet (mean reduction
26 per cent) and least for cereals (mean reduction 7 per cent).Yield reductions did not extend further
than 20m from the field margin. They attributed these losses to soil compaction from machinery as it
turned. When hedges were part of the field margins yield losses were greater due to shading. Areas
shaded by trees produced 4.4 t ha-1 of wheat as compared to non shaded areas which produced 8.1 t
ha-1.Yields of wheat were halved within 9m of the hedge and the grain contained slightly more moisture.
The authors suggested that the area within 20m of the hedge was a good candidate for set-aside.
6.3.2 Pasture
Russell and Grace (1979) conducted an experiment over three years in Scotland on the effect of artificial
shelter on pasture yield. Wind speeds were reduced by 27 per cent which resulted in increases in dry
matter yield in the re-growth period, but not in spring. However, they could not attribute the increases to
leaf water potential but did not rule it out as a possibility. They concluded that the most likely explanation
was that sheltered conditions reduced leaf damage, which can increase evapotranspiration losses.
A study at Pontbren showed that deciduous tree shelterbelts planted on grazed lands could reduce
flood risk (Bird et al., 2003; Carroll al., 2006). Within two years of planting infiltration rates were higher
than non-sheltered areas up to 5m from shelter. Infiltration rates were up to 60 times higher than
non-tree planted areas.
28
6.4 Agroforestry studies
UK agriculture may in dry windy conditions benefit from shelter, but yields may be reduced at field
margins. The effect of shelterbelts on crop yield is a balance between positive and negative interactions.
There are a number of agroforestry experiments that have examined these interactions. These are
relevant as the findings can give insight to how shelterbelts may compete with crops. Most of the
experiments were part of two UK initiatives that investigated yields of tree/pasture (Silvopastoral national
network) and tree/arable systems (SAFE : Silvoarable Agroforestry for Europe) (Agroforestry Forum,
not dated). The trees were planted in alleys and at various levels of spacing.
6.4.1 Pasture
Shading
Northern European agricultural production is largely limited by light and southern European agricultural
production by water (Eichhorn et al., 2006). Bergez et al. (1997) conducted an experiment at three sites
in the UK to examine the effect of tree shading on pasture production. Light levels were measured under
the stands of eight year old sycamore, ash and larch trees. The light levels intercepted by the tree canopies
were quite low; on average tree shading reduced the amount of light reaching
the sward by 8 per cent.
The introduction of trees did not reduce the animal carrying capacity of the pasture.
The authors suggested a number of explanations for this result some of which were: maximum sward
growth occurred before the leaves of the tree were fully expanded; shelter afforded by trees may have
conserved sward soil water moisture; livestock had reduced energy expenditure due to shelter.
McEvoy and McAdams (2008) conducted an experiment in Northern Ireland examining the effect of
sheep grazing on ash and oak pastures. Prior to the introduction of sheep, swards were taller under ash
than oak pastures. These results could be explained by the fact that ash leaves are only fully expanded
after 70 per cent of pasture growth has occurred and their canopies are smaller than oak (QMS, 2010).
Silvopastoral farming system at Loughall: © Paul Burgess
Wilcox et al. (2000) studied the effect of crop margins bordered with 1.5 to 2m high hedges on arable
crop yields at Shropshire (Harper Adams), Hampshire and Leicestershire.Yields were reduced up to
30m into the field at one site, but this could not be attributed solely to the presence of weeds. The
authors concluded that yield reductions were site dependent and could be due to presence of weeds,
seed bed quality and soil compaction. They made no comment on the effect of the hedge.
UK evidence
29
Managing the drought
UK evidence
Root competition
Trees occupy the same competitive root space as crops. However, when confronted with competition
tree and crop roots can change their behaviour. Campbell et al. (1994) studied root competition of wild
cherry trees inter cropped with pasture at a site in Scotland under varying levels of N application. The
study showed that grass was more competitive than trees. Only under hot dry conditions and at the
highest applications of nitrogen did water limit tree growth.
Dawson et al. (2001) using the same study site as Campbell a few years later studied the distribution of
tree and grass roots. Tree roots avoided the top 5cm of soil which was occupied by grass roots. Tree roots
were only found in the top 5cm of soil if grass growth was suppressed by herbicide treatment.
Silvoarable barley farming system: © Paul Burgess
Soil properties
As discussed earlier Carroll et al. (2006) found that shelterbelts increased infiltration rates. Parkinson et
al. (1998) studied the hydrology of an ash agroforestry site in Devon. Water drained quicker from the soil
in the alley cropped site than the site without trees. The authors suggested that this was due to enhanced
soil macroporosity created by tree root exploration.
Microclimate study
A series of microclimate measurements were made at a conifer agroforestry site in Scotland (Sibbald et
al., 1991; Sibbald and Griffith, 1992; Sibbald et al., 1994; Greens et al., 1995). Some general principles can be
gained from these experiments which can have some extrapolation to native deciduous trees. Wind speed
reduction was measured at different densities of conifer planting. Wind speed was measured 2m above the
ground and was found to be 46 per cent, 29 per cent and 16 per cent in the wide, medium and narrow
plots, respectively (Greens et al., 1995). At the greatest tree density, grass meristem temperature was
reduced (Sibbald and Griffith, 1992). Herbage yield declined with tree density.Yield reductions were not
significant if the trees were less than 8m tall and at a spacing of more than 6m.The authors suggested
that trees may promote some out of season pasture growth (Sibbald et al., 1991). As part of the same
study it was found that the horizontal projection of tree crowns gave a good indication of herbage yield
(Sibbald et al., 1994).
6.4.2 Arable
Agroforestry experiments that have mixed trees and arable have focused on the yield of both crop and
tree. There has been a large focus on poplar trees which are fast growing and produce marketable timber
relatively quickly (Burgess et al., 2003; Burgess et al., 2005).
Effects of water competition and shading
Burgess et al. (2003, 2005) conducted a large scale agroforestry study using young poplar trees.
Poplars were inter-cropped with various arable crops and growth rates were compared to a poplar fallow
control. Arable crops caused a reduction in tree growth as compared to the control. The poplar and
arable treatment had a greater soil water deficit than the control suggesting that water was limiting tree
growth. Crop yield was reduced by10 per cent, due to either shading, competition for water, weeds or
slug damage. Trees had the greatest effect on break crops and the least effect on winter cereals suggesting
that shading was limiting crop growth. The yield of inter cropped mustard and beans was about 80 per
cent as compared to pure arable field, whereas, inter cropped winter wheat yields were about 93 per cent
of the arable field. Winter crops finish much of their development before tree leaves emerge, therefore
avoiding the adverse effects of shading. Burgess et al. (2005) reported that high pruning of poplars
minimized the effect of shading on crop growth.
Nutrient inputs
Ranasinghe and Mayhead (1990) studied the effect of inter cropping poplars and beans as compared to a
poplar control. They found that even at two years old the greatest tree growth was at the widest spacing.
There were higher levels of nitrogen in the poplars grown with beans as compared to the control.
At the end of the second year bean growth was reduced within 25cm of the tree due to reduced light and
moisture. The authors concluded that the poplars had benefited from growing next to the beans due to
the increased N levels.
Park et al. (1994) investigated soil properties of arable inter-cropped with four year old poplars.
They found that within 1m of the tree, there was more organic matter and soil invertebrates. The authors
concluded that poplars were unlikely to have a detrimental effect on soil organic matter.
30
6.5 Computer modelling
Part of the SAFE project was to develop a computer model to predict yields of both crops and trees over
time (Van der Werf et al,, 2007). The experimental data to validate the model was derived from the poplar
experiments reported by Burgess et al. (2003). The model consists of seven equations to express tree
biomass, tree leaf area, number of shoots per tree, crop biomass, crop leaf area index, soil water content
and heat sum. The model takes as daily input parameters temperature, radiation and precipitation.
Planting densities, soil parameters and initial biomasses of tree and crop can also be specified as
parameters. The authors did not indicate if wind speed was also an input. For the first 12 years the model
generally matched the experimental results.
31
European evidence
The benefits of native trees species for shelter on the water
regime of pasture and arable crops
7 European Evidence
7.1 Introduction
Many examples of traditional agricultural practices that grew crops and trees together have now fallen
out of practice in Europe (Eichhorn et al, 2006). At one time large areas of central Europe had fruit
and nut silvoarable systems (Sinclair, 1999). Since Roman times, pigs have been released into beech and
oak woodlands to eat acorns and beech mast, a practice called pannage. In northern Europe mature
woodlands provided shelter to livestock in winter (Smith, 2010).
Shelterbelts are found on the Jutland plains of Denmark and the steppes of Russia and Ukraine
(Hislop and Claridge, 2000). Hedgerows still cover many areas of Europe such as in the Rhone Valley,
where a series of hedges have been planted for water conservation. A 50m hedgerow can store
between 150 and 375 cubic metres of water, which can be released during dry periods. In north West
France, hedges were removed from the bocage region which resulted in soil erosion. Regulations now
stipulate that hedges have to be planted in these areas (Merot, 1999; Hedgelink, 2009).
Farmers in drier parts of Europe are more likely to plant trees (Dupraz et al., 2005). Three million
hectares of mainly oak mixed with pastures or crops cover parts of Spain and Portugal, a practice called
dehesa (Lawson et al., 2004). The dehesa provide shelter for livestock and the wood is used for timber.
In Italy olives and vines are inter cropped with cereals.
7.2 Shelterbelt studies
7.2.1 Microclimate
Campi et al. (2009) conducted a three year study of rain fed durum wheat growing in Italy. A 3m high
cypress shelterbelt sheltered the crop from the prevailing winds. Trees were 20 years old and had
a porosity of 40 per cent. Wind speed was reduced at a distance of 12.7H from the shelterbelt and
temperature was increased up to a distance of 4.7H from the shelterbelt. Evapotranspiration was
reduced up to a distance of 12.7H from the shelterbelt. The greatest efficiency in water use was found
at 2.7H and at 4.7H the evapotranspiration rate was 16 per cent lower than unsheltered areas.Yields
were increased within a distance of 18H of the shelterbelt. The authors suggested that shelterbelts
could be considered as an agricultural practice for sustainability.
Trees providing a windbreak in Denmark: © Poula Hansen/iStock
32
Foerid et al. (2002) conducted a study in Denmark of a four year old hazel coppice shelterbelt growing
with barley. The shelterbelt reduced winds by about 50 per cent. Generally air and soil temperature
were increased close to the shelterbelt during the day and slightly decreased at night. Soil moisture
and humidity were slightly higher nearer the shelterbelt. The authors stated that water was never
limiting. Light levels were reduced by about 10 per cent. Barley growth was slightly reduced close to the
shelterbelt and the authors suggested this was due to evening shading. Anthesis and crop development
were slightly earlier closer to the shelterbelt due to increased temperatures. The computer model
Sirus was run with crop growth and climatic data. The model validated that temperature and light levels
accounted for these results. The authors did not report on overall barley yields.
Jossart et al. (1998) studied a short rotation willow coppice grown with maize in Belgium. Shelter had
an effect on the speed of maize development, but not yield. These studies show that bioenergy crops
can be grown within an arable crop system.
33
Managing the drought
European evidence
7.3 Agroforestry studies
Orfanus and Eitzinger (2010) studied the effect of soil texture and 8m hedge shelter on the growth of
alfalfa in one of the windiest and driest parts of central Europe, the Austrian basin. Evapotranspiration
was significantly lower at 20m (2.5H) from the hedge than at 80m (10H) due to reduced wind speed.
Sandy soil had to have 8 per cent more water compared to clay soils to eliminate plant water stress.
They concluded that hedges can save almost 2000 m3 ha-1 of water due to reductions in wind speed
(presumably over a year).
Balandier et al. (2008) studied ten year old cherry trees inter-cropped with pasture in the Auverne,
France. Tree and grass roots avoided the same competitive space. Grass roots occupied the upper 20cm
layers and tree roots the lower 20-80cm layers. Trees grown with grass were more water stressed,
possibly due to grass roots extracting water from the top 20cm of topsoil as it fell as rain.
De Montard et al. (1999) studied the effect of competition for light, water and nitrogen between hazel
and cocksfoot growing in Clermont Ferrand, France. Hazel was very water stressed when grown with
cocksfoot. However, cocksfoot growth was not limited by shading and during wet windy periods it
benefited from the presence of trees. After four years of planting, grass roots were confined to the top
0.2m of soil. The authors suggested this was due to the hazel drying lower levels of soil and therefore
the grass avoided that soil layer. Picon-Cochard et al. (2001) also found that walnut seedling growth was
inhibited by presence of grass, whereas the grass was not affected by the presence of the walnut
7.2.2 Large scale effects of shelterbelts
Ryszkowski and Kędziora (1987, 2008) studied the effects of deciduous and conifer shelterbelts on
the hydrology and energy fluxes of large areas of the Turew landscape in Poland. They showed that the
structure of the vegetation is very important for energy balances and thus evapotranspiration rates.
The total energy in a landscape is affected by the incoming solar radiation and the reflection of that
energy from vegetative surfaces. Mixtures of cereals, row crops and shelterbelts reflected the lowest
amount of radiation, whereas meadows reflected the most. Wheat fields used three times more energy
heating air than shelterbelts.
Silvopastural system: © Paul Burgess
Shelterbelts had the highest rates of evaporation and meadows had the lowest. Shelterbelts evaporated
more water and used 40 per cent more energy for evapotranspiration than wheat. Shelterbelts acted
as water pumps humidifying and cooling the landscape resulting in lower crop evapotranspiration
losses in adjoining fields. The authors estimated that landscapes with a 20 per cent cover of deciduous
shelterbelts could conserve considerable amounts of water and this effect would be most pronounced
under hot dry conditions. These findings are in agreement with the study of the water balances of the
steppes of Northern Caucasus by Lazarev (2006). They showed that shelterbelts increased rainfall by
22 per cent and reduced soil water run-off. Soil water evaporation from bare soil was reduced by
34 per cent. They estimated that the annual water input into the soil increased by 3.5 times.
These studies show that shelterbelts can have a profound impact on water regimes.
7.3.1 Studies reported in literature reviews
Hedges in Brittany can also have a big impact on hydrology.Viaud et al. (2005) showed that hedges
could increase annual evapotranspiration rates from 5 to 30 per cent. The groundwater table was
lowered suggesting that hedge roots were able to uptake water from this resource.
Nuberg (1998) cited work from Germany, Norway, Czechoslovakia, Ukraine and Russia in a review of
the effects of shelter on arable yields. Pretzschel (in German) studied the effect of shelterbelts on the
productivity of arable crops over six years. The study found that the benefits of shelter depended on
the crop. Potatoes responded well to shelter with a yield increase of 29.7 per cent, whereas barley
mixed with oats had the least yield increase. Nuberg (1998) also cited an experiment by Miloserdov
(in Russian) conducted in the Ukraine. An analysis of 25 years of data showed that shelterbelts
increased barley yields. The effect was most marked in dry years when net revenues were increased by
27–52 per cent. Another Ukrainian study by Tkach (in Russian) showed that particularly in windy years
shelter increased the yield of wheat.
Landscape of Brittany: © Pere Sanz/iStock
34
Kort (1988) after reviewing 50 years of research concluded that shelterbelts increased yields. Included
in that review were studies from Germany, Denmark, Poland, Ukraine and Russia. Most of the studies
reported an area of reduced growth between 0.5H and 1.5H from the shelterbelt, but these losses
were compensated for by increased yield in the area between about 1.5H – 7H. Kort (1988) stated that
yield reductions were dependant on the competitiveness of the tree and the crop.
Bird (1998) found that there was a lack of research on the effects of shelter on pasture. Only two
western European studies were cited, that of Russell and Grace from the UK and a study by Alterna
(in Dutch) from the Netherlands. Alterna found that pasture yields were reduced by 10 per cent either
side of a shelterbelt comprised of oak and birch. The authors attributed this loss to animal poaching and
tree pasture competition.
35
Temperature zone evidence
The benefits of native trees species for shelter on the water
regime of pasture and arable crops
8 Temperate zone evidence
8.1 Introduction
Shelterbelts can be found in Canada, USA, Australia, New Zealand, China, Argentina and many
developing countries (Campi et al., 2009). National Shelterbelt programmes have been set up in Canada,
USA, China and Australia to promote tree planting (Brandle et al., 2004). The Agriculture and Agri-Food
Canada Prairie Shelterbelt Program has created a large network of shelterbelts across the prairies
of Canada. The shelterbelt programme is one of the longest running government programmes.
Since 1901, 600 million tree seedlings have been grown and given to 700,000 farms (AAFC, not dated a;
AAFC, not datedb; Wiseman et al., 2009).
This section will look at evidence from other temperate zones for the beneficial effects of shelterbelts
on the water regime of pasture and arable crops. It will examine evidence from North American
prairies, silvopastoral systems of New Zealand and the shelterbelts of temperate Australia.
8.2 North America
8.2.1 Canada
The Agri-Food Canada Prairie Shelterbelt website states that shelterbelts on the prairies can increase
wheat yields by 3.5 per cent and that yield increases are higher in drier years. The site recommends
that shelterbelts should only take up 5 per cent of land and that trees should be selected with upright
growth to avoid sprawling branches (AAFC, not dateda).
Kowalchuk and Jong (1995) studied the effect of a shelterbelt comprising green ash and maple on yields
of wheat, barley at a site in Saskatoon, Canada.Yield increases were most pronounced when water
supply was low. The shelterbelt reduced yields up to 2H into the field, but in seasons when water was
not limiting this zone was smaller. Increased yields were observed between 2H and 4H into the field.
Wind speed was reduced by 40 per cent at 1H and the evapotranspiration rate was 75 per cent of
the non-sheltered site at the same distance. The authors concluded that increased yields were due to
reductions in crop evapotranspiration rates.
Windbreaks sheltering crops in New Zealand: © Andrew Simpson/iStock
36
Pelton (1967) studied the effect of a 2.5m high artificial shelter of 45 per cent porosity on yields
of wheat. The fence reduced wind speed by between 15 per cent and 50 per cent and resulted in
reductions in evapotranspiration of between 12 per cent to 23 per cent.Yields were increased from
between 23 per cent to 43 per cent. The beneficial effects were strongest up to a distance of 10H from
the shelter. The authors noted that yield increases were highly variable from year to year.
Bayou (1997) studied the effect of a hedge on the microclimate of a field. The study found that
microclimate was not consistently modified by the hedge due to variability in hedge porosity.
The author suggested that management programmes should be in place to ensure that hedgerows
have the optimum porosity to maximise the benefits of shelter.
37
Managing the drought
Temperature zone evidence
8.2.2 USA
8.3 New Zealand and Australia
In the prairies of the USA, shelterbelts were planted to reduce wind erosion (Mize et al., 2008a).
Bates conducted pioneering research into the effects of shelterbelts on microclimate and crop yields in
Nebraska in 1911. He found that up to 12H from the shelterbelt there was a reduction in water loss of
12-36 per cent (Bird, 1998).
8.3.1 New Zealand
New Zealand has a tradition of mixed tree pasture systems as it is a windy country (Benavides et
al., 2009; Mead, 1995). Bird (1998) compiled a literature review of the effect of shelter on pasture
production citing two New Zealand studies. One was the work of Radcliffe who studied the effect on
the growth rate of pasture of a shelterbelt comprised of pine and cedar. Pasture production at 3H to
5H from the shelterbelt was 6.5t as compared to a midfield value of 4t.Yield was 3.6t at 0.3H from the
shelterbelt. The shelterbelt resulted in a 60 per cent increase in pasture production which the authors
concluded was due to reduced evapotranspiration rates. Bird (1998) also reports on the work of
Hawke and Tombleson who found pasture growth was increased at 0.7H from the shelterbelt, possibly
due to nutrient transfer from grazing animals.
Asse and Sidway (1974) studied tall wheat grass barriers sheltering wheat in Montana. They found that
wheat growth benefited most from shelter in dry years suggesting that shelter benefits were not just
restricted to snow capture, but also included reductions in crop evapotranspiration rates.
Lyles et al. (1984) studied the effect of root pruning on wheat next to a shelter of deciduous trees in
Kansas. Wheat yields in the 0.5h to 2H zone were 1.6 times higher in the pruned site as compared to
the non-pruned site. Soil water content was lower nearer the shelterbelt for the non-pruned site and
this effect was strongest in summer and autumn. The study suggested that root pruning can reduce
water competition between the crop and tree shelter. Hou et al. (2003) studied the effect of root
pruning on yields of soya beans next to a shelter of either conifer or deciduous trees in Nebraska.
Soil water content was higher at 0.75H in the pruned treatment, as compared to the non-pruned
treatment. The increases in yield at the pruned site was matched by an increases in soil water content.
After 1H from the shelter there was no difference between treatments. Both studies suggest that the
observed reductions in yield were due to competition for water.
Wall et al. (2010) and Wall (2006) looked at the viability of growing poplars in pastures under New
Zealand conditions. If canopy closure was prevented from getting more than 30-40 per cent then
pasture production could be maintained to 75 per cent of that of open pasture. The authors suggested
that if pruning and thinning was maintained, then the negative effects of shading could be reduced.
Poplar trees also increased the soil pH, therefore exerting a beneficial effect on nutrient cycling.
8.3.2 Australia
Shelterbelts can also reduce water loss from spray irrigation and therefore increase crop water
efficiency (Bird, 1998). Dicky (1988) in the USA reported that shelterbelts reduced irrigation water
losses by as much as 42 per cent.
Australia is a country that suffers from extremes of weather conditions with severe droughts.
Shelterbelts offer the potential to alleviate this water stress, however wind speeds are not as strong
or in such a consistent direction as New Zealand. Oliver et al. (2005) cites Australian studies that
have shown a positive effect of shelter on yields. One study by Bicknell showed yield increases
in lupins and oats in the lee of a pine shelterbelt. Bird (1998) reported on the work of Lynch and
Donnely which demonstrated that artificial shelter could increase pasture production due to reduced
evapotranspiration rates.
Gillespie et al. (2000) studied maize growing with black walnut or red oak. When maize and tree roots
were isolated, there was no reduction in maize yield. The authors concluded that shading had no
negative effect on maize production.
A computer model called the Erosion-Productivity Impact Calculator (EPIC) was developed in
Nebraska by Easterling (1997). Simulations demonstrated that shelter gave the most benefit to crops
when conditions are hot and windy. The authors suggested that shelterbelts could help combat the
adverse effects of climate change.
Harvesting wheat in Whitman County: © Phil Augustavo/iStock
38
The National Windbreaks programme was started to address the lack of shelterbelt research in
Australia. Some yield increase was demonstrated, but results were not convincing as wind direction
was inconsistent resulting in fields not always being sheltered. A significant yield increase was only
demonstrated when wind direction was consistent. Shelterbelts had no effect on ground water levels.
The authors concluded that shelterbelts were only appropriate for certain sites (Cleugh et al., 2002;
Sudmeyer and Scott, 2002).
Nuberg and Mylius (2002) studied wheat growing near Adelaide with an artificial shelterbelt. Sheltered
plants were 7 per cent more efficient in water use. Early in the season less water was lost from the
soil in the sheltered site. Sheltered wheat conserved water early in the growing season and was more
efficient in biomass production, but some of the saved water was used to support the extra biomass.
There was no overall increase in grain yield under sheltered conditions. The authors commented that
for farmers growing biomass crops shelter could be advantageous.
A large scale study in southern Australia looked at 32 different tree formations to investigate whether
it was better to integrate, or separate trees for water resource management (Oliver et al., 2005).
Climatic effects had a large effect on results; some years it was better to have trees mixed with crops
whilst other years not. A combination of factors could make the inter cropping more productive such
as access to perched water tables, fields with consistent wind directions, trees aged less than ten years
and low rainfall (less than 400mm).
39
Discussion
The benefits of native trees species for shelter on the water
regime of pasture and arable crops
9 Discussion
9.1 Benefits of shelter
Studies in Europe and other temperate zones have the shown that shelterbelts can reduce wind speeds
up to 30H from the shelterbelt. Crops and trees compete for water and light. This effect frequently
disappears at a distance of 1 to 2H from the shelter belt, at which point yield increases may occur.
The area at a distance of 1.5 to 9 times the height of the shelterbelt produces the greatest yield
increases. Therefore based on these values it can be hypothesised that a 10m high shelterbelt of
between 40-60 per cent porosity could reduce wind speeds over an area of 300m from the shelter.
Yield reductions due to tree crop competition could be confined to within about 15m of the
shelterbelt. The area between 15 and 90m from the shelter could have the greatest crop yields.
The effects of shelter can vary with year, crop, soil type, shelterbelt design, stage of crop development
and geography (Kort, 1988; Bird, 1998). Bird (1998) stated that finding the particular factor which
produced the yield increase can be difficult and quoted Davis and Norman (1988) who stated that
‘shelter has multiple, interacting effects on crop processes which almost defy explanation’.
This review has presented evidence from temperate agricultural systems which suggest that
shelterbelts can have a positive effect on water regimes. This effect is more pronounced under hot,
dry conditions. Shelter can also effect plant growth sometimes increasing crop biomass or reducing
mechanical damage. However, the introduction of trees into an agricultural system can result in
competition for resources.
9.1.1 Shelterbelt and crop water regime
Farmers in drier areas of Europe are more inclined to plant trees (Dupraz et al., 2005). Herzog (2000)
stated that agricultural production was limited in northern Europe by light and in southern Europe
by water. However, under climate change scenarios parts of northern Europe will experience water
shortages. It has already been suggested that Mediterranean crop trials could start in southern England
(Thompson et al., 2007).
A study in Italy by Campi et al. (2009) showed that a shelterbelt could reduce rain fed durum wheat
evapotranspiration rates and increase yields up to 12H from shelter. Likewise, Orfanus and Eitzinger
(2010) found that in one of the driest parts of central Europe, evapotranspiration rates of alfafa were
reduced at 20m from an 8m high hedge. Canadian studies have also shown that shelterbelts can reduce
crop evapotranspiration losses and increase crop yields (Pelton 1967; Kowalchuk and Jong, 1995).
Sheep resting in the shade of a native tree: © Duncan Payne/iStock
40
The benefits of shelterbelts become more pronounced when the plants are water stressed and wind
direction is consistent. Shelter induced yield increases of wheat and barley were only observed in the
UK in the years when the weather was hot and dry (Hough and Cooper, 1988). Asse and Sidway (1974)
also found that wheat growth benefited most from shelter in the drier years.
In contrast, a study in Denmark by Foerid et al. (2002) observed no yield increase with a shelterbelt of
a hazel coppice, but the barley crop was not water stressed. In Australia shelterbelts have been used to
protect crops against drought. Despite the hot conditions a study in Australia showed no positive effect
of shelter on crop growth, as a result of inconsistent wind direction (Cleugh et al., 2002; Sudmeyer and
Scott, 2002).
41
Managing the drought
Large networks of shelterbelts can also reduce evapotranspiration rates of crops by modifying local
air humidity. In Poland large networks of shelterbelts act as water pumps cooling the air. Trees have
high evapotranspiration rates, which humidify the environment reducing evapotranspiration crop
losses (Ryszkowski and Kędziora, 2008). Shelterbelts can also exert a beneficial effect on soil structure
increasing soil infiltration rates as demonstrated at Pontbren, Wales (Carroll et al., 2006). Within two
years of tree planting soil water infiltration rates were increased.
Foerid et al. (2002) in Denmark and Nuberg and Mylius (2002) in Australia both found that shelterbelts
increased the amount of biomass produced by a crop. However, in both cases this did not result in
higher grain yields. Shelter can also reduce mechanical damage to plants, reducing water loss from
damaged tissue (Russell and Grace, 1979).
9.1.2 Tree crop interactions
Agroforestry experiments conducted in the UK have demonstrated that crops and native deciduous
trees can be grown together and that by judicious mixing of crop and trees, economic returns can be
maintained. (Sibbald et al., 2001; Burgess et al., 2005). Bergez et al. (1997) found that the introduction of
trees into grazed pastures did not reduce the animal carrying capacity of the field. However, Sparkes et
al. (1998) found that crop yields were reduced near the field edge due to shading. Careful selection of
trees can reduce the negative effects of shelter. Deciduous trees are better than conifers as they have a
leafless period which allows pastures to recover in winter from the adverse effects of shading.
Trees and crops can avoid being in the same competitive rooting space (De Montard et al., 1998;
Balandier et al., 1994; Dawson et al., 2004). Studies in the USA have shown that the zone of reduced
yield next to a shelterbelt can be decreased by pruning tree roots (Lyles et al., 1984; Hou et al., (2003).
9.2 Shelterbelts in the UK
Shelterbelts laid out in the 19th century in Scotland have been neglected and their management for
multiple purposes has resulted in them losing their effectiveness (Palmer et al., 1997). A new initiative
between the Forestry Commission and Quality Meat Scotland is now promoting the use of shelterbelts
on Scottish farms. Livestock derive benefit from shelter gaining relief from cold winters and hot
summers (QMS, 2010; QMS, 2011a; QMS, 2011b).
Shelterbelts can provide many ecosystem functions reducing the risk of soil erosion, water logging and
flooding (Nisbet et al., 2011). In addition, they can increase biodiversity, promote crop pollination from
insects and improve the aesthetic value of landscapes. They can produce timber to supplement farm
income and reduce green house gas emissions (Cleugh et al., 2002). An environmental stewardship
scheme report found that hedges provided 19 ecosystem functions (DEFRA, 2009).
9.3 Benefits of shelter on the water regime of crops
The evidence in this review suggests that under the right conditions shelterbelts can enable crops
to use water more efficiently. Table 2 summarizes some of the observed effects of shelter. In the UK
there has been little research covering the benefits of shelterbelts on crop water regimes. However,
studies from drier parts of the world have shown that shelterbelts are beneficial when wind direction
is consistent and water is limiting to plant growth. To be effective shelterbelts should be of the correct
porosity and have a suitable tree crop mix. Shelterbelts composed of native deciduous trees are a good
choice as they are well adapted to local conditions and have a leafless period which can reduce the
negative impacts of shading on crop growth.
42
Discussion
Based on the evidence presented in this review shelterbelts could counteract the negative effects
of heat waves and droughts that climate change scenarios suggest. Shelterbelts can be viewed as an
insurance policy, they may not provide yield increases every year, but they can buffer crop production
when extreme weather events strike (Sudmeyer et al., 2007). Droughts are predicted to become more
common place in the UK and shelterbelts could have a beneficial effect on the water regimes of crops
and pastures. As Caborn wrote in 1955 “From this evidence it should be apparent that there is a place
in the rural economy for shelter-belts both on arable and hill farms” this is even more relevant today
when considering the threats to UK food security from climate change.
Author
Country
Tree/Crop/Formation
Effect of shelter
Orfanus and
Eitzinger (2010)
Austria and Slovakia Hedge/Pasture/Shelter
Evapotranspiration rate reduced
Campi et al. (2009)
Italy
Conifer/Arable/Shelter
Evapotranspiration rate reduced
Kowalchuk and Jong,
(1995)
Canada
Deciduous/Arable/Shelter
Evapotranspiration rate reduced
Pelton (1967)
Canada
Artificial/Arable/Shelter
Evapotranspiration rate reduced
Ryszkowski and
Kędziora (2008)
Poland
Deciduous/Arable/Shelter
Deciduous/Pasture/shelter
and some conifers
Air humidified
Hough and Cooper
(1988)
UK
Artificial/Arable/Shelter
In drier years yield increase
Asse and Sidway
(1974)
USA
Tall wheat/Arable/Shelter
In drier years yield increase
Nuberg and Mylius
(2002)
Australia
Artificial/Arable/Shelter
Increase in water efficiency, but
not grain yield
Foerid et al. (2002)
Denmark
Deciduous/Arable/Shelter
Earlier anthesis, increase biomass,
but not grain yield
Russell and Grace
(1979)
UK
Artificial/Pasture/Shelter
Reduced mechanical damage
Yield increase regrowth period
Carroll et al. (2006)
UK
Deciduous/Pasture/Shelter
Soil infiltration rates increased
Dawson et al. (2001)
UK
Artificial/Pasture/Alley
Roots avoided same competitive
space
Balandier et al.
(2008)
France
Deciduous/Pasture/Alley
Roots avoided same competitive
space
De Montard et al.
(1999)
France
Deciduous/Pasture/Alley
Roots avoided same competitive
space
Hou et al. (2003)
USA
Deciduous/Arable/Shelter
Root pruning increased yield, and
soil water content
Lyles et al. (1984)
USA
Deciduous/Arable/Shelter
Root pruning increased yield
Bergez et al. (1997)
UK
Deciduous/Pasture/Alley
8 year old trees incepted 8% of
the light
Less energy available to drive ET
Table 2
Main conclusions from the major shelter studies found in the literature.
43
Managing the drought
10 References
AAFC Agriculture and Agri-Food Canada), not dateda. Increasing Crop Yield with Shelterbelts.
www4.agr.gc.ca/AAFC-AAC/display-afficher.do?id=1192556664605&lang=eng
[Accessed August 18, 2011].
AAFC (Agriculture and Agri-Food Canada), not datedb. Planning Field Shelterbelts. Available at: www4.agr.gc.ca/AAFC-AAC/display-afficher.do?id=1192546880508&lang=eng [Accessed September 8, 2011].
Agroforestry Forum, not dated. Silvopastoral Agroforestry Toolbox. Available at:
www.macaulay.ac.uk/agfor_toolbox/manage.html#pastures [Accessed September 1, 2011].
Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. 1998. Crop evapotranspiration-Guidelines for computing crop water
requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300, p.6541.
Aase, J. and Siddoway, F. 1974. Tall wheatgrass barriers and winter wheat response. Agricultural Meteorology, 13(3),
p.321-338.
Balandier, P., Montard, F. and Curt, T, D. 2008. Root competition for water between tres and grass in a silvopastoral plot of
10 year old prunus avium. In Ecological basis of agroforestry eds (Batish, D). Boca Raton FL: CRC Press.
Baldwin, C.S. 1988. 10. The influence of field windbreaks on vegetable and specialty crops. Agriculture, Ecosystems &
Environment, 22-23, p.191-203.
Bayou, M.M. 1997. The effect of natural fencerows on local standardized windspeed, temperature and relative humidity. PhD.
University of Toronto. Available at: https://tspace.library.utoronto.ca/bitstream/1807/11737/1/MQ29455.pdf
[Accessed August 17, 2011].
Benavides, R., Douglas, G.B. and Osoro, K. 2009. Silvopastoralism in New Zealand: review of effects of evergreen and
deciduous trees on pasture dynamics. Agroforestry systems, 76(2), p.327-350.
Bergez, J.E., Dalziel, A.J.I., Duller, C., Eason, W., Hoppe, G. and Lavender, R. 1997. Light modification in a developing
silvopastoral system in the UK: a quantitative analysis. Agroforestry systems, 37(3), p.227-240.
Bird, P. 1998. Tree windbreaks and shelter benefits to pasture in temperate grazing systems. Agroforestry systems, 41(1),
p.35-54.
Bird, S.B., Emmett, B.A., Sinclair, F.L., Stevens, P.A., Reynolds, B., Nicholson, S. and Jones, T. 2003. Pontbren: Effects of
tree planting on agricultural soils and their functions. Available at:
www.ceh.ac.uk/sections/bef/Pontbren_report.html [Accessed August 17, 2011].
Blyth, E., Evans, J., Finch, J., Bantges, R. and Harding, R. 2006. Spatial variability of the English agricultural landscape and
its effect on evaporation. Agricultural and forest meteorology 138(1-4), p.19-28.
44
References
CALU, 2006. Shelterbelt Information Sheet, Available at:
www.calu.bangor.ac.uk/Technicalleaflets/050204shelterbelt info sheet.pdf [Accessed August 15, 2011].
Campbell, C.D., Atkinson, D. and Newbould, P. 1994. Effects of nitrogen fertiliser on tree/pasture competition during the
establishment phase of a silvopastoral system. Annals of applied biology, 124(1), p.83-96.
Campi, P., Palumbo, A.D. and Mastrorilli, M. 2009. Effects of tree windbreak on microclimate and wheat productivity in a
Mediterranean environment. European Journal of Agronomy, 30(3), p.220-227.
Carroll, Z.L., Bird, S.B., Emmett, B.A., Reynolds, B. and Sinclair, F.L., 2006. Can tree shelterbelts on agricultural land
reduce flood risk? Soil Use and Management, 20(3), p.357-359.
Chandler, K.R. and Chappell, N.A. 2008. Influence of individual oak (Quercus robur) trees on saturated hydraulic
conductivity. Forest Ecology and Management, 256(5), p.1222-1229.
Charlton, M.B., Bailey, A. and Arnell, N. 2010. Water for Agriculture - Implications for Future Policy and Practice, Available
at: www.rase.org.uk/pdfs/Water_Report-Final.pdf [Accessed September 1, 2011].
Cleugh, H. 1998. Effects of windbreaks on airflow, microclimates and crop yields. Agroforestry Systems, 41(1), p.55-84.
Cleugh, H. and Hughes, D. 2002. Impact of shelter on crop microclimates: a synthesis of results from wind tunnel and field
experiments. Australian Journal of experimental agriculture, 42(6), p.679-702.
Cleugh, H., Prinsley, R., Bird, R. P., Brooks, S. J., Carberry, P. S., Crawford, M. C., Jackson, T. T., Meinke, H., Mylius, S. J.,
Nuberg, I.K., Sudmeyer, R. A. and Wright, A. J. 2002. The Australian National Windbreaks Program: overview and
summary of results. Australian Journal of Experimental Agriculture, 42(6), p.649.
Dawson L.A., Duff E.I., Campbell C.D. and Hirst, D.J. 2001. Depth distribution of cherry (Prunus avium L.) tree roots as
influenced by grass root competition. Plant and soil, 231(1), p.11-19.
Davis, J.E. and Norman, J.M. 1988. 22. Effects of shelter on plant water use. Agriculture, Ecosystems and Environment 2223, 393-402.
DECC (Department Energy and climate change) not dated. Wind Speed Database. Available at: www.decc.gov.uk/en/windspeed/default.aspx [Accessed August 16, 2011].
DEFRA (Land Use Consultants in association with GHK Consulting Ltd), 2009. Provision of ecosystem services through
the environmental stewardship scheme Final report, Available at:
www.hedgelaying.ie/images/1253357089.pdf [Accessed September 1, 2011].
DEFRA 2010a. UK Climate Projections - Reports & guidance: Updates. Available at:
http://ukclimateprojections.defra.gov.uk/content/view/720/500/ [Accessed August 15, 2011].
DEFRA 2010b. Defra’s Climate Change Plan 2010. Available at: www.defra.gov.uk/publications/files/pb13358-climatechange-plan-2010-100324.pdf [Accessed August 15, 2011].
Brandle, J.R., Hodges, L. and Zhou, X.H. 2004. Windbreaks in North American agricultural systems. Agroforestry systems,
61(1), p.65-78.
DEFRA 2011. Water Usage in Agriculture and Horticulture Results from the Farm Business Survey 2009/10 and the
Irrigation Survey 2010. Available at: www.defra.gov.uk/statistics/files/defra-stats-foodfarm-farmmanage-fbs-
waterusage20110609.pdf [Accessed August 15, 2011].
Breadalbane Initiative, not dated. Breadalbane Initiative for Farm Forestry. Available at:
www.breadalbanefarmforestry.co.uk/silvo-pasture/index.htm[Accessed August 17, 2011].
Dickey, G.L. 1988. 21. Crop water use and water conservation benefits from windbreaks. Agriculture, Ecosystems and
Environment, 22-23, p.381-392.
Brouwer, C, Goffeau, A and Heibloem, M. 1985. Irrigation Water Management:Training Manual No. 1 - Introduction to Irrigation.
Irrigation Water Management:Training Manual No. 1 Introduction to Irrigation: Chapter 2 - Soil and Water. Available at:
www.fao.org/docrep/R4082E/R4082E00.htm [Accessed August 16, 2011].
Burgess, P.J., Incoll, L.D., Corry, D.T., Beaton, A. and Hart, B.J. 2005. Poplar (Populus spp) growth and crop yields in a
silvoarable experiment at three lowland sites in England. Agroforestry systems, 63(2), p.157-169.
Dupraz C., Burgess P., Gavaland A., Graves A., Herzog F., Incoll L., Jackson N., Keesman K., Lawson G., Lecomte
I., Liagre F., Mantzanas K., Mayus M., Moreno G., Palma J., Papanastasis V., Paris P., Pilbeam D., Reisner Y.,
Vincent G., Werf Van der W.,, 2005. SAFE final report, Available at:
www.ensam.inra.fr/safe/english/results/final-report/SAFE Fourth Year Annual Report Volume 2.pdf
[Accessed August 17, 2011].
Burgess, P.J., Incoll, L.D., Hart, B.J., Beaton, A., Piper, R.W., Seymour, I., Reynolds, F.H., Wright, C., Pilbeam and Graves,
A.R. 2003. The Impact of Silvoarable Agroforestry with Poplar on Farm Profitability and Biological Diversity Final
Report to DEFRA. Project Code: AF0105. Silsoe, Bedfordshire: Cranfield University., Available at: http://
randd.defra.gov.uk/Document.aspx?Document=AF0105_1062_FRP.doc
[Accessed September 1, 2011].
Easterling, W. 1997. Modelling the effect of shelterbelts on maize productivity under climate change: An application of the
EPIC model. Agriculture, Ecosystems and Environment, 61(2-3), p.163-176.
Eichhorn, M., Paris, P., Herzog, F., Incoll, L., Liagre, F., Mantzanas, K., Mayus, M., Moreno, G., Papanastasis,V., Pilbeam,
D., Pisanelli, A. and Dupraz, C. 2006. Silvoarable Systems in Europe – Past, Present and Future Prospects.
Agroforestry Systems, 67(1), p.29-50.
Caborn, J.M. 1955. The influence of shelter-belts on microclimate. Quarterly Journal of the Royal Meteorological Society,
81(347), p.112-115.
FAO 1989. Chapter 2: Environmental links between forestry and food security. Available at:
www.fao.org/docrep/T0178E/T0178E03.htm [Accessed August 15, 2011].
Caldwell, M.M., Dawson, T.E. and Richards. J.H 1998. Hydraulic lift: consequences of water efflux from the roots of plants.
Oecologia 113:151–161.
Foereid, B., Bro, R., Mogensen,V.O. and Porter, J.R. 2002. Effects of windbreak strips of willow coppice-modelling and field
experiment on barley in Denmark. Agriculture, Ecosystems and Environment, 93(1-3), p.25-32.
45
Managing the drought
Forestry Commission Scotland, not dated. Farm Woodlands - Planning your tree planting. Available at: www.forestry.gov.uk/forestry/infd-8aejun [Accessed August 31, 2011].
Gardiner, B, 2004. Windbreaks and Shelterbelts. In Encyclopedia of Forest Sciences. Oxford: Elsevier, pp. 1801-1806.
Gardiner, B., Palmer, H and Hisplop, M, 2006. The Principles of Using Woods for Shelter, Available at: www.forestry.gov.uk/pdf/fcin081.pdf/$FILE/fcin081.pdf [Accessed September 1, 2011].
Ghazavi, G., Thomas, Z., Hamon,Y., Marie, J.C., Corson, M. and Merot, P. 2008. Hedgerow impacts on soil-water transfer
due to rainfall interception and root-water uptake. Hydrological Processes, 22(24), p.4723-4735.
Gillespie, A., Jose, S., Mengel, D., Hoover, W., Pope, P., Seifert, J., Biehle, D., Stall, T. and Benjamin, T. 2000. Defining
competition vectors in a temperate alley cropping system in the midwestern USA: 1. Production physiology.
Agroforestry systems, 48(1), p.25-40.
Glensaugh, not dated. Agroforestry at Glensaugh The James Hutton Institute. Available at: www.hutton.ac.uk/about/facilities/glensaugh/agroforestry [Accessed August 17, 2011].
Gloyne, R.W. 1976. Shelter in agriculture, forestry and horticulture - a review of some recent work and trends. ADAS
Quarterly Review, 21, p.197-207.
Lawson, G., Burgess, P., Crowe, R., Mantzanas, K., Mayus, M., Moreno, G., McAdam, J., Newman, S., Pisanelli, A. and
Schuman, F. 2004. Policy support for agroforestry in the European Union, Available at: www1.montpellier.
inra.fr/safe/english/results/final-report/D9-3.pdf
[Accessed September 1, 2011].
Lazarev, M.M. 2006. Transformation of the annual water budget of soils under shelterbelts. Eurasian Soil Science, 39(12),
p.1318-1322. [Online abstract]. CAB Abstracts. Available from: http://cababstracts.edina.ac.uk
Liste H.H. and White J.C. 2008 Plant hydraulic lift of soil water: implications for crop production and land restoration. Plant
and Soil 313 p.1-17.
Lyles, L.,Tatarko, J. and Dickerson, J. 1984. Windbreak effects on soil water and wheat yield.Transactions of the ACAE, 30, p.30-5.
McEvoy, P. and McAdam, J. 2008. Sheep grazing in young oak Quercus spp. and ash Fraxinus excelsior plantations:
vegetation control, seasonality and tree damage. Agroforestry systems, 74(2), p.199-211.
Mead, D.J. 1995. The role of agroforestry in industrialized nations: the southern hemisphere perspective with special
emphasis on Australia and New Zealand. Agroforestry systems, 31(2), p.143-156.
Grace, J. 1988. 3. Plant response to wind. Agriculture, Ecosystems and Environment, 22-23, p.71-88.
Merot, Philippe 1999. The influence of hedgerow systems on the hydrology of agricultural catchments in a temperate
climate. Agronomie 19(8), p.655-669.
Greens, S.R., Grace, J. and Hutchings, N.J. 1995. Observations of turbulent air flow in three stands of widely spaced Sitka
spruce. Agricultural and Forest Meteorology, 74(3-4), p.205-225.
Mize, C, Colletti, J., Batchelor, W., Kim. J., Takle, E and Brandle, R., D. 2008b. Modeling a field shelterbelt system with the
shelterbelt modeling system. In Ecological basis of agroforestry eds (Batish, D). Boca Raton FL: CRC Press.
Hedgelink, 2009. The importance of hedgerows and the services they provide to society. Key messages. Available at: www.hedgelink.org.uk/hedgelink/importance-hedges-and-hedgerows.htm [Accessed August 15, 2011].
Mize, C., Brandle, J.R., Schoeneberger, M. and Bentrup, G. 2008a. Ecological development and function of shelterbelts in
temperate North America.Toward Agroforestry Design, p.27-54.
Hein, S. and Spiecker, H. 2008. Crown and tree allometry of open-grown ash (Fraxinus excelsior L.) and sycamore (Acer
pseudoplatanus L.). Agroforestry systems, 73(3), p.205-218.
Herzog, F. 2000. The importance of perennial trees for the balance of northern European agricultural landscapes, Available at: ftp://ftp.fao.org/docrep/fao/x3989e/x3989e07.pdf [Accessed September 1, 2011].
De Montard, F.X., H. Rapey, H., Delpy, R. and Massey, P. 1998. Competition for light, water and nitrogen in an association
of hazel (Corylus avellana L.) and cocksfoot (Dactylis glomerata L.). Agroforestry systems, 43(1), p.135-150.
Morison, J.I.L., Baker, N.R., Mullineaux, P.M. and Davies, W.J. 2008. Improving water use in crop production. Philosophical
Transactions of the Royal Society B: Biological Sciences, 363(1491), p.639-658.
Hislop, A.M and Claridge, J. (eds) 2000. Agroforestry in the UK, Edinburgh: Forestry Commission.
Murphy, J.M., Sexton, D.M.H., Jenkins, G.J., Boorman, P.M., Booth, B.B.B., Brown, C.C., Clark, R.T., Collins, M., Harris,
G.R., Kendon, E.J., Betts, R.A., Brown, S.J., Howard, T. P., Humphrey, K. A., McCarthy, M. P., McDonald,
R. E., Stephens, A., Wallace, C., Warren, R. and Wilby, R., Wood, R. A. (2009), UK Climate Projections
Science Report: Climate change projections. Met Office Hadley Centre, Exeter. Available at: http://
ukclimateprojections.defra.gov.uk/[Accessed August 17, 2011].
Hislop, A.M., Gardiner, B.A. and Mochan, S. 1997. The shelterbelts of Lothian:The work of Maurice Caborn 40 years on.
Scottish Forestry 51, p.199-205.
Natural England, not dated. North Kent Plain. Available at:
www.naturalengland.org.uk/Images/jca113_tcm6-5301.pdf [Accessed August 17, 2011].
Hooper, M. and Holdgate, M. 1968. Hedges and hedgerow trees Monks Wood Symposium No. 4 held 25th - 26th
November 1968.
Natural England, 2008. Hedgerow trees : answers to 18 common questions, Natural England. Available at:
http://naturalengland.etraderstores.com/NaturalEnglandShop/NE69[Accessed August 18, 2011].
Hough, M.N. and Cooper, F.B. 1988. The effects of shelter for cereal crops in an exposed area of Cleveland, North-East
England. Soil Use and Management, 4(1), p.19-22.
Nelmes, S., Belcher, R.E., Wood, C.J. 2001. A method for routine characterisation of shelterbelts. Agricultural and Forest
Meteorology, 106(4), p.303-315.
Hou, Q., Brandle, J., Hubbard, K., Schoeneberger, M., Nieto, C. and Francis, C. 2003. Alteration of soil water content
consequent to root-pruning at a windbreak/crop interface in Nebraska, USA. Agroforestry systems, 57(2), p.137-147.
Nisbet, T. 2005. Water Use By trees. Available at: www.forestresearch.gov.uk/fr/infd-6mvj8b [Accessed August 2, 2011].
Hislop, A.M., Palmer H.E. and Gardiner, B.A. 1999. Assessing woodland shelter on farms. In Burgess, P. (ed) Farm
woodlands for the future : papers presented at the conference “Farm Woodlands for the Future”,
Cranfield University, Silsoe, Bedfordshire, UK, 8-10 September 1999. Oxford:BIOS.
Jose, S., Gillespie, AR and Pallardy, S. 2004. Interspecific interactions in temperate agroforestry. Agroforestry systems, 61(1),
p.237-255.
Jossart, J.M., Goor, F., Ledent, J.F. 1998. Short rotation coppice: Shelterbelt effects. Biomass for Energy and Industry, pp.
857-859. [On-line abstract]. CAB Abstracts. Available from: http://cababstracts.edina.ac.uk [Accessed 2
August 2011].
Kettlewell, P.S. 2010. Yield enhancement of droughted wheat by film antitranspirant application: rationale and evidence.
Agricultural Sciences, 1(03), p.143-147.
Knox, C, Morris, J. and Hess,T. 2010. Identifying future risks to UK agricultural crop production. Outlook on Agriculture,
39(4), p.245-248.
Kort, J. 1988. 9. Benefits of windbreaks to field and forage crops. Agriculture, Ecosystems and Environment,
22-23, p.165-190.
Kowalchuk, T. and De Jong, E. 1995. Shelterbelts and their effect on crop yield. Canadian Journal of Soil Science, 75(4),
p.543-550.
46
References
Nisbet, T., Silgram, M., Shah, N., Morrow, K., and Broadmeadow, S. (2011) Woodland for Water:Woodland measures for
meeting Water Framework Directive objectives. Forest Research Monograph, 4, Forest Research, Surrey, 156pp
Available at: www.forestresearch.gov.uk/pdf/FRMG004_Woodland4Water.pdf/ [Accessed August 18,2011].
Nuberg, I. 1998. Effect of shelter on temperate crops: a review to define research for Australian conditions. Agroforestry
systems, 41(1), p.3-34.
Nuberg, I. K. and Mylius, A.S. 2002. Effect of shelter on the yield and water use of wheat. Australian Journal of Experimental
Agriculture, 42, p.773–780.
Oliver,Y. Lefroy,T., Stirzaker, R. Davies, C. and Waugh, D. 2005. Integrate, Segregate or Rotate Trees with Crops?
Measuring the trade-off between yield and recharge control. A report for the RIRDC/Land and Water
Australia /FWPRDC/MDBC Joint Venture Agroforestry Program, Available at:https://rirdc.infoservices.
com.au/downloads/04-177.pdf Accessed August 18, 2011].
Orfánus,T. and Eitzinger, J. 2010. Factors influencing the occurrence of water stress at field scale. Ecohydrology, 3(4), p.478-486.
Palma, J., Graves, A., Bunce, R., Burgess, P.J., Keesman, K., Liagre, F., Mayus, M., Moreno, G., Reisner,Y. and Herzog,
F. 2007. Modeling environmental benefits of silvoarable agroforestry in Europe. Agriculture, Ecosystems and
Environment, 119, p.320-334.
47
Managing the drought
Palmer, H., Gardiner, B., Hislop, M, Sibbald,A. and Duncan,A. 1997. Trees for shelter, Edinburgh: Forestry Commission.
Parkinson, R., Williams, A., Penn, M. and Schofield, D. 1998. Temporal and spatial patterns of soil water in a silvopastoral
production system. In proceedings of 16th World Congress of Soil Science Montpellier France: Available at:
natres.psu.ac.th/Link/SoilCongress/bdd/symp14/258-t.pdf [Accessed September 1, 2011].
Park, J., Newman, S.M. and Cousins, S.H. 1994. The effects of poplar (P. trichocarpa deltoides) on soil biological properties
in a silvoarable system. Agroforestry systems, 25(2), p.111-118.
Pelton, W. 1967. The effect of a windbreak on wind travel, evaporation and wheat yield. Canadian Journal of Plant Science,
47(2), p.209-214.
Picon-Cochard, C., Nsourou-Obame, A. and Collet, C., 2001. Competition for water between walnut seedlings (Juglans
regia) and rye grass (Lolium perenne) assessed by carbon isotope discrimination and 18O enrichment.Tree
Physiology, 21(2-3), p.183.
QMS(Quality Meat Scotland), 2011b. Report on meeting at Auchmacoy Estate, Ellon 23 February 2011. Available at:
www.qmscotland.co.uk/index.php?option=com_remository&func=startdown&id=850
[Accessed August 16, 2011].
QMS(Quality Meat Scotland), 2011a. Report on meeting at Bargany Estate, 2nd March 2011. Available at:
www.qmscotland.co.uk/index.php? option=com_remository&func=startdown&id=847
[Accessed August 15, 2011].
QMS (Quality Meat Scotland), 2010. Report on Meeting at Bolfracks Estate 15th December 2010. Available at: www.
qmscotland.co.uk/index.php? option=com_remository&func=startdown&id=790 [Accessed August 10, 2011].
References
Thompson, J A., King, K.A., Smith, H. and Tiffin 2007. Opportunities for Reducing Water use in Agriculture Final Report,
WU0101, Available at: http://randd.defra.gov.uk/Document.aspx?Document=WU0101_5888_FRA.
doc[Accessed August 8, 2011].
Tuzet, A. and Wilson, J.D. 2007. Measured winds about a thick hedge. Agricultural and forest meteorology, 145(3-4), p.195-205.
Van der Werf,W., Keesman, K., Burgess, P., Graves,A., Pilbeam, D., Incoll, L., Metselaar, K., Mayus, M., Stappers, R., van Keulen,
H., Palma, J., and Dupraz, C., 2007.Yield-SAFE: A parameter-sparse, process-based dynamic model for predicting resource
capture, growth, and production in agroforestry systems. Ecological Engineering, 29(4), p.419-433.
Viaud,V., Durand, P., Merot, P., Sauboua, E. and Saadi, Z. 2005. Modeling the impact of the spatial structure of a hedge
network on the hydrology of a small catchment in a temperate climate. Agricultural Water Management, 74(2),
p.135-163.
Vigiak,O.,Sterk,G. Warren,A. and Hagen,J. 2003. Spatial modeling of wind speed around windbreaks. Catena, 52(3-4),
p.273-288.
Wall, A. 2006. The effect of poplar stand density on hill country pastures. PhD thesis. Institute of Natural Resources,
Massey University, Palmerston North, New Zealand. Available at: http://mro.massey.ac.nz/handle/10179/1517. [Accessed September 8, 2011].
Wall, A.J., Kemp, P.D., Mackay, A.D. and Power, I.L. 2010. Evaluation of easily measured stand inventory parameters
as predictors of PAR transmittance for use in poplar silvopastoral management. Agriculture, Ecosystems and
Environment, 139(4), p.665-674.
Ranasinghe, O. and Mayhead, G. 1990. The Effect of Intercropping Populus “RAP”with Beans. Forestry, 63(3), p.271.
Weatherhead, E.K. and Howden, N.J.K. 2009. The relationship between land use and surface water resources in the UK.
Land Use Policy, 26, p.S243-S250.
RHS (Royal Horticultural Society), not dated. Windbreaks and shelterbelts. Available at: http://apps.rhs.org.uk/
advicesearch/Profile.aspx?pid=624#section3 [Accessed August 16, 2011].
Wilcox, A., Perry, N.H., Boatman, N.D. and Chaney, K. (2000). Factors affecting the yield of winter cereals in crop
margins. Journal of Agricultural Science, 135 p. 335-346.
Riha, S.J and McIntyre, B.D 1999. Chapter 3. Water Management with Hedgerow Agroforestry Systems. I
n Sulzycki, J,Startkweather, A. and Boyd, D. (ed). Agroforestry in sustainable agricultural systems.
Wiseman, G., Kort, J. and Walker, D. 2009. Quantification of shelterbelt characteristics using high-resolution imagery.
Agriculture, Ecosystems & Environment, 131(1-2), p.111-117.
Russell, G. and Grace, J. 1979. The effect of shelter on the yield of grasses in southern Scotland. Journal of Applied Ecology, p.319-330.
Ryszkowski, L. and Kdziora, A. 1987. Impact of agricultural landscape structure on energy flow and water cycling.
Landscape Ecology, 1(2), p.85-94.
The Woodland Trust, not dateda. A Guide to Creating Small Native Woods in England. Available at: www.woodlandtrust.org.uk/en/planttrees/help-advice/Documents/creating-small-scale-native-woodland.
pdf [Accessed September 5, 2011].
Ryszkowski, L. and Kedziora, A. 2008. 11 The Influence of Plant Cover Structures on Water Fluxes in Agricultural Landscapes,
Available at: www.iwmi.cgiar.org/publications/CABI_Publications/CA_CABI_Series/Conserving_Land_
Protecting_Water/protected/9781845933876.pdf [Accessed September 1, 2011].
The Woodland Trust, not datedb. Tree planting and woodland creation for farms. Available at: www.woodlandtrust.org.uk/en/campaigning/our-views-and-policy/agriculture/Documents/trees-for-farmsdocument.pdf [Accessed September 6, 2011].
Sharrow, S. 1999. Competition and Facilitation Between Trees, Livestock, and Improved Grass-Clover Pastures on Temperate Rainfed
Lands. In Sulzycki, J,Startkweather, A. and Boyd, D., (ed.) Agroforestry in sustainable agricultural systems.
Sibbald, A., Eason, W., McAdam, J. and Hislop, A. 2001. The establishment phase of a silvopastoral national network
experiment in the UK. Agroforestry systems, 53(1), p.39-53.
Sibbald, A. and Griffiths, J. 1992. The effects of conifer canopies at wide spacing on some understorey meteorological
parameters. In Agroforestry Forum. pp. 8-15.
Sibbald, A., Griffiths, J. and Elston, D. 1994. Herbage yield in agroforestry systems as a function of easily measured
attributes of the tree canopy. Forest Ecology and Management, 65(2-3), p.195-200.
Sibbald, A., Griffiths, J. and Elston, D. 1991. The effects of the presence of widely spaced conifers on under-storey herbage
production in the UK. Forest Ecology and Management, 45(1-4), p.71-77.
Sinclair, F.L. 1999. A general classification of agroforestry practice. Agroforestry systems, 46(2), p.161-180.
Smith, J. 2010. The History of Temperate Agroforestry, Available at: orgprints.org/18173/1/History_of_agroforestry_
v1.0.pdf [Accessed September 1 2011].
Sparkes, D.L., Jaggard, K.W., Ramsden, S.J. and Scott, R.K. 1998. The effect of field margins on the yield of sugar beet and
cereal crops. Annals of Applied Biology, 132(1), p.129-142.
Sudmeyer, R.A. and Scott, P.R. 2002. Characterisation of a windbreak system on the south coast of Western Australia. 1.
Microclimate and wind erosion. Australian Journal of Experimental Agriculture, 42(6), p.703.
Sudmeyer, R., Bicknell, D. and Coles, N. 2007.Tree windbreaks in the wheatbelt. Bulletin, 4723. Available at: www.agric.
wa.gov.au/content/lwe/land/bulletin07_windbreaks.pdf[Accessed September 1, 2011].
48
49
Managing the drought
Appendix
11 Appendix
General
Wind speed
Microclimate
Tree/crop
Databases
Websites
Agroforestry
Shelter
Microclimate
Water competition
Google Scholar
Forestry Commission/Forestry Research
Silvopasture
Wind speed
attenuation
Evapotranspiration
Water uptake
Web of Science
The Woodland Trust
Soil evaporation
Water requirements
PhD theses UK thesis
Centre for Ecology and Hydrology
Tree-Pasture
Wind speed
reduction
Plant transpiration
Water use
Science Direct
Natural England
Tree-arable
Wind break
Shading Light interception
Water regime
Open fields
Countryside Council for Wales
Forest edge
Shelterbelt
Heat advection
Tree placement
IngentaConnect
Scottish Natural heritage
Hedgerow
Wind Shelter
Radiation
Tree density
Oalster
ADAS
Orchard
Wind flow
Air/Soil temperature
Tree spacing
Cam Abstracts
Farm woodland forum
Field margin
Aerodynamics
Vapour pressure deficit
Tree age
Campaign for the farmed environment
Agroecosystem
Humidity
Root length
Scottish Environment Agency
Parkland
Vapour transfer
Root architecture
European Environment Agency
Silvoarable
Temperate
National Farmers Union
Native broadleaved
RSPB
Deciduous
Shropshire Hills
Irrigation
Drought
Climate change
Precipitation
UK
England
Wales
Scotland
Britain
Ireland
Europe
Canada
USA
New Zealand
50
Table 1
Table 2
Keywords used in literature search.
Literature sources.
51
Report edited, designed and printed by the Woodland Trust
The Woodland Trust, Kempton Way, Grantham, Lincolnshire NG31 6LL.
The Woodland Trust is a charity registered in England and Wales no. 294344 and in Scotland no. SC038885.
A non-profit making company limited by guarantee. Registered in England no. 1982873.
The Woodland Trust logo is a registered trademark.
Front cover image: Wheat harvest in Hampshire © Peter Dean/Agripicture Images.
Printed on recycled paper Á 5208 03/12
52